I haven't seen anyone address the awkwardness felt by remote employees, but believe this is critical to understanding the dynamic at play.
Its about context
I work remotely much of the time. When working on a project in my spare time I don't feel a need to constantly prove myself on "official priority" type items, but instead pick whatever I think advances the project (and my) goals the most. That might mean writing documentation, or even working on tools or language resources that aren't directly a part of the project (writing a tutorial on a strongly related API, or some necessary but otherwise obscure technique on a personal website, for example -- this is doubly true in the semi-invisible not-quite-FOSS community dev landscape in Japan). I can't get fired, feel no stress here and am doing things because I want to. I regularly wind up in off-topic chat with people I've met in various language or project communities, and occasionally gotten to know a few folks well who continue to stay in touch with me long after one of us has split from a given community.
Its different at work, though, especially remote work. There is usually a subtle, constant pressure in the back of one's mind to "prove" that you're "really working" when its all remote. Different teams subconsciously give off a different vibe on this point. Working remotely feels less secure in this way most of the time -- and that is because it really is less secure. Working remotely you are far more likely to be let go than if you walk in to the same building as the CEO every day unless you're the only guy who knows how to manage critical system X. That is to say, unless you're the reason that the bus factor approaches 1 working remotely is always going to feel a lot less secure than working in person. Working remotely still often feels like a privilege (even in 2016) -- not only the one that feeds your family but also (for many) the one that allows you to even live with your family.
Given the difference between the two contexts, its scary to be seen in "idle chat" instead of constantly making commits or reaping tickets or whatever -- even on trivial busywork or contrived issues -- because the stakes for being taken for a slacker are so high when the work is remote and you actually depend on that job. These effects are doubled when there is a sense that the management views workers as interchangeable components of an organizational lego figure than as unique individual people who each have their own way to contribute to the company's effort and independent take on the company's vision and future.
Why would people get that feeling? Consider what happens when an eager worker is told, however politely, to "stay in their lane". On an open source project if you really think the cause of low adoption is a lack of documentation about feature X, you don't passively drop hints about it in IRC -- you do something if you have the time and inclination. You start writing documentation for feature X, blog about it, give a presentation about it, write a book about it, talk about it, and otherwise try to attract attention to the project and the great-but-hitherto-unwieldy feature. I've seen many cases in work environments, however, where a lowly minion observed that product adoption was low because of a lack of documentation or some other peripheral resource that was not their direct responsibility, and were rebuffed when they expressed an interest in improving this area of the customer/user experience.
When this happens in-person the blow of rejection is cushioned a good bit by the body language that frames the reaction. When this happens remotely it is very easy to take a direct text sentence in chat from a manager or team leader the wrong way -- or worse, not really know which way to take it. In the best case the worker may feel that they are being told to stick to a higher company priority based on demonstrated skillset, at worst they may begin to fear that the rejected suggestion made them appear to be distracted, uninterested in their own work or otherwise a bad fit for the team in the mind of the manager. The common lack of constant interpersonal reassurance in remote work can make people gradually feel more awkward over time without a bit more stroking ("lack" here being lack of facial contact, body language, casual-but-positive encounter in the hallway, etc.).
Now back to the watercooler thing. How much time is enough? How much time is too much? How burned out are your workers already? How well do they get along? How well can they get along? What private interests, present or past, intersect among various workers? You can't know any of these things, so there are definite limits to trying to force watercooler moments. There is a fine line between "mandatory fun" and "additional workload".
The "Approved Tools" dictate the environment
I haven't used Slack (yet), but have used Google Hangouts, Skype, IRC, Campfire, some homebrew systems, and a few other services in remote companies. The places where informal, spontaneous chat among remote workers was most common were in flexible, ephemeral systems like Skype, IRC, one chat system that was actually based on a MUD (for real -- also, surprisingly effective), etc.
Because in such systems it is easy to ping any arbitrary number of people for ephemeral, unlogged, boss-invisible communication, and doing so was seen as integral to the work process, not something that needed to be controlled and channelized by the management. Systems like Campfire and Google Hangout company accounts were the least effective at inspiring spontaneous communication. I'm not quite sure why, but part of it was that they were things I only interacted with for serious business and they were logged forever as company property. Jokes that were funny 10 years ago when the team was 10 people may later be regarded as coarse or radically insensitive -- values dissonance can be a tricky thing in a long-lived, geographically diverse team. Do we really want to log every snarky comment that may lead to productive banter about Issue X?
Logging in to the VPN, showing my presence on my "work" Skype account, changing my activity timer on Campfire, changing my status thingy on Google Hangouts, etc. was a matter of proving that I was actually actively participating in work that day. Even installing Google Hangouts feels silly when its something you never would have used personally if given the choice. I've spoken with other folks at one particularly poisoned environment I worked at in the past about this and they had the same feeling: flipping the presence indicators on all the officially sanctioned and mandatory "social" tools was a ritual that prepared one to exist in Big Brother Is Watching mode. This sort of puts a damper on "off-topic" (aka "genuine") social interaction across official channels.
I don't know that there is a solution to this, because I don't know that there is really a problem. That said...
As a leader, when I need some social glue to fill in the gaps of interpersonal cohesion among teammembers I both try to generate it myself as much as possible (sometimes tricky) and also try to find signs of it emerging naturally and capitalize on them. Show interest in some peripherally related issue someone makes a side-comment about, even if you're just batting back; chances are someone else is interested in it, too. This is a delicate art, almost like trolling, but the idea is to demonstrate by example that The Management isn't opposed to a bit of side chatter and expose the interests of some team members to the rest of the group. In the proper environment (reference the wall of text I wrote above) the natural chemistry between people will eventually bring them into their own social orbits. You have to keep an eye on all that, but not too close an eye or you risk spoiling the soup.
As with almost anything, there is no recipe for success in this area but there are many formulas for failure.