Online friendships are a lot like offline friendships. There are those who are friends of yours for life regardless of geography. And there are those who are friends of yours for a season for any number of reasons such as being coworkers, friends of friends, etc.
I always find it interesting that some folks on Facebook show up continuously in comments, Likes, postings on your Wall for a period of time and then join the Witness Relocation Program. And then at some point down the road they show up hot and heavy again. And then they disappear for months. Or years. And then they show up again.
There’s nothing wrong with that. Just invest your time and attention in those who hang around.