What happened in May 2007?

Someone else already mentioned Facebook getting more popular (i.e., taking up time). I just looked at its Timeline and I think that's quite likely to be an explanation of at least part of what happened. That timeline also mentions MySpace, which was also getting popular at that time. I think Bebo in the UK was also popular. There's also "and then the novelty wore off" explanation. 'First adopters' move on, same as 'entrepreneurs'. It's what they do. They love 'new development', they hate 'maintenance'. Which is why, imo, it's not going to work to keep trying to attract that same sort of contributor. They're not interested, and we're turning off every other sort of contributor.