An elderly but still relevant and very perceptive short piece, pre the rise of Facebook, on why social networks don't always succeed. The reasons the article gives are:

1. privacy - or the lack of it
2. no real reward or penalty system - what's the point of sharing, where's the value?
3. not granular enough - we don't have binary relationships - they have a context and vary according to what our current context might be (e.g. a workmate who is also a friend)
4. not integrated with other apps - predates Chris Anderson's distinction of social networks as destination or feature and speaks to the importance of mashability
5. walled gardens - not connected, isolated pools

I like these a lot, and they reflect a number of the issues I and many others have been writing about since and that strongly underpin Athabasca Landing, this site - not bad for 2006. But the interesting thing here is that Facebook then happened, largely by concentrating on the single second aspect of value and not just ignoring the other four issues but actually riding rough-shod over them and squashing them into the ground. Which is of course why I hate Facebook, because these principles are sound, usable and relate to human, not multi-billion-dollar company needs and interests. However, it shows that you can probably do away with a lot of 'necessary' things if you do one thing very well and, equally, that there are quite a few necessary things that are not mentioned here.