I don’t know.
I do know one thing: it’s a very emotionally charged word. It is certainly distinct (though has a connection to) “being trusted”. There are people I trust who I do not consider friends, and my friends are trusted to varying degrees. It is certainly distinct from “fun to talk to”: there are people who are interesting and witty whom I do not consider friends, and my friends are interesting and witty to varying amounts. And yet, this emotionally charged word, with all the (by me) poorly understood meaning of it, is the word Livejournal uses to conflate the other two meanings with (trust and interest). Facebook is even worse: not only it conflates it with trust (people who can see parts of my profile) and interest (people I get notifications about) but it also tells me it must be reciprocative, so these other people must care about and trust me — and it seems this is the trend in social networks.
I do not think the solution is just to change the word used, but also to completely dissociate the terms. Friends do not need to be defined, so social networks can just stay away from that term. Then I’d add the concept of “share lists” (people you want to share stuff with — the most naive implementation can be just one share list, while you could also do full set-theoretic implementation with “share this item only to people who both have proper classification and need to know about my life”) and “watch lists” (people whose stuff you want to watch, with implementations again ranging from naive to “hard to find UI for”).
There are many variants — send notifications to people when they get on/off a list (WITHOUT implying reprocity is intended — “Moshe has chosen to share with you ‘programming’ items”), allow people to share/not share the share/watch lists, etc. It’s possible to make the UI suggest common things (“You have added Moe to ‘filk’ share. Click HERE to also add him to the ‘filk’ watch list.”) without any invariant being forced. I could share something with Moe but not watch him (I trust him, but most of his stories about filk are just boring anecdotes about the guitar strings he buys), I could watch Moe but not share filk with him (he’s a famous interesting filker, but there is no need he should be privy to my private discomfort with writing lyrics), and none of these suggest to Moe he’s the scum of the earth if he does not share/watch me.