I had the same problem. I started a tiny InvisionFree board and got about 50 members. I pretty much stopped visiting it for about a month and a half to work on the website to go with it. When I took a break from the website to visit the board, I found that one person had blabbed about MySpace and a bunch of people had read it and just about all the membership had mass-migrated to MySpace and all my moderators had either gone with them or just sat there feebly and then mustered enough energy to resign.
I was left with myself, one admin who promptly became a devout Christian and comdemned my board (it was a Harry Potter board), another admin who simply pretended that nothing had happened and bravely posted things that no one read, a third one who simply pretended that the board didn't exist, and one who simply disappeared into thin air and couldn't be reached by any means.
All because of MySpace and Yahoo and places like them.
Also, the ideas in this thread were good. I tried to register on MySpace so I could direct traffic to my site and vBulletin board, but it said that I was "ineligible to register". I think it had something to do with the image verification. I'm not good with stuff like that because they don't show the letters clearly. I mean, if a computer can't read the picture, why do they have to make it murky?
I disagree. Probably about a million of those members are only there to try and regain their lost members, who are the other 39 million.