I've tried a few of these social networks, like ecademy and linkedin. I've been under-enthused, mostly because of the sense that many people there are networking for networking's sake.

But Connectviabooks feels a little more promising. Enter the books you like, and it will point you to other people who are into the same books. It strikes me that people's choice of books is going to provide some clues to some of their deeper passions and interests. There are a few books I've read where I'd be genuinely interested in meeting the sort of people who also enjoyed them. So I'm giving it a go, and I think this site is onto something worthwhile.

Bonus link: Jyri Engestrom gave a great talk at Reboot about the need for an object for social networks to flourish. Here's his account of it.