Most of them, I'm sure, wound up there the way I did: some casual acquaintance emailed me, asking me to join, and figuring I had nothing to lose, I signed on. A few more people found me and that was that.
There are a few people who really work Linked In, people who have several hundred "links." But by and large the magic number seems to be somewhere south of 10. So much for networking sites, huh?