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I recently received a friend request on Facebook from one of my managers. My initial reaction was to reject this out of hand, however, this person is of a comparable age to me and at a similar place in life, so I think it's reasonable to consider that we could be friends outside the office as well.
The caveat, however, is that I'm aware of my own social media persona and demeanor outside the workplace. I don't want to say I'm unprofessional, but I'm definitely more brash and way more politically active.
As a result, I'm concerned that adding my manager could create issues, but ignoring or deleting the request seems likely to invite issues as well.