I have no reason to call them
Networking (, The art of) is not an endeavour you always undertake for immediate or medium-term gain. You don't need some life-threatening emergency to reach out to someone. LinkedIn, through it's endorsement and sharing of articles gives an easy way to stay on a contact's radar for no cogent reason. Think long-term. I presume you're familiar with the HTTP protocol as a developer. You know what happens to a HTTP session when the user doesn't do anything for a while? It dies. And this is what will happen to all those links the business cards and chin-wagging you engage in at conferences. You need to find a way to chip in a hello (read: ping) how's the state of "The Business"(whatever that may be for your contact) every couple of weeks/months or you'll just fade into the ether.
I can't remember the faces to the names, nor most of what we discussed.
This is your problem then. To sort this out, I'd recommend you start by striking up conversation on something you're actually interested in. Not B.S-ing your way through a topic or subject you couldn't give a flying dutchman about. Also pick your targets carefully, in the same vein. 3 business cards from people you actually have stuff in common with will serve you better (and be easier on your memory) than a fistful of business cards from people you barely have anything in common and you're going to forget as soon as you turn heel.
No one wants to invite me to any evening events, and if I suggest a evening meet-up, they always either have other plans or just want to relax alone
This might be a combination of not having anything in common with your acquaintances, hanging in the wrong circle or coming off as not being genuine enough. I meet someone and it seems (s)he's only hanging out with me for superficial reasons or for what he can get out of me, I bob and weave until we fall out of touch. Unless I need something from him too. "Let's be friends first" eh? Genuineness goes a long way with me. Just today I made a professional contact and after the first 15 minutes of conversation, the dude dropped a couple of colourful words and I was sure we'd connected there. I'm not saying you should go all Samuel L. Jackson on your contacts, but try to loosen up a bit, let them see a bit of the real you.
I never seem to meet anyone who can help me with anything
Again, think long term. And try not to be grabby. You can't expect to mine a contact for all possibilities after the first meet. Typically you'll only get to second base after the 2nd-3rd meet. Impatience sets in when you're expecting short-term turnaround on a contact (see my first point) and you have too many of them in the first place. Pace yourself and invest yourself in someone you have a solid link with.
So to summarize,
Find a medium (Not too invasive) that lets you stay on your contact's radar and possibly pop in some pleasantries from time to time. Ideally, such a medium should also let you stay abreast of your contact's activties (only in a way that concerns you anyway) and let's you share your interests with your contact. Yes, I've basically described social networking. LinkedIn has a great set of features (Sharing articles, endorsing skills etc) that allow you to do just this in a minimally intrusive manner
At your conferences, don't shoot for quantity. Shoot for quality contacts, people you share interests with, with whom you have a connection. Relax and be yourself (to a large an extent as possible without looking unprofessional)
Lose the "I'm here to meet people with whom I'll have a mutually beneficial relationship starting next week" outlook. Meet this people and give the contact time to mature and bear good fruit