David Weinberger delivers some well-placed low blows against social networking software.

real social networks are always implicit...Explicitly constructed social networks not only lack the differentiation that makes relationships real, they are falsehoods built to reinforce spectral relationships and to avoid ending shaky ones.

Moreover, relationships are not transitive, as David points out. I know that I interact with Alpha and with Beta on a regular basis. This creates trust in a number of ways. However, when Alpha meets Beta, they do not know how many interactions that they will have in the future. So they cannot possibly relate to one another the way that I relate to them.

I suspect that social networking software would be more valuable in working backwards than in working forwards. That is, what is interesting is not working from someone I know to someone I do not know, but rather working from someone I just met to someone I know.

How often do you meet someone, find out something about their background (say, that they went to a certain college from 1983-1986), and ask, "Oh, do you know so-and-so?" I could see using social software to do that sort of thing.