Eons ago I joined Livejournal in order to yell at someone in an academic community. Not much has changed since.
Despite the fact that I mainly use my LJ for griping about academia in communities, I DO sometimes post to my own journal. Historically I've kept friends to a very, very small number of people whom I know in real life or with whom I've had extensive interaction in communities. While I don't foresee that this will change, if we have interacted a fair bit and you'd still like to be friends... well, sir or madam, you need better taste in friends.