Talking to people on OS, it does have some to do with the new child avatar rules but not the nudity part of it or the adult sims. Some of them felt they were being singled out as second-class citizens and decided they have had enough. The prevailing thought is that second life is trying to get rid of child avatars so they just moving on. Not all of them are child avatars ether. There is a lot that don't like the implications that was brought up with the new privacy policy that seems to be in the works. That line that Kieria Linden dropped about proactive moderation really raised some eyebrows. Besides a great many grids don't' allow child avatars, period so there is more to it than just the child av policies at play here.
Economics are playing parts of it too. Some of them left SL because of its high cost and opensims are cheaper, much cheaper in some cases.
Some furries are concerned that after SL rids the grid of child avatars they are next on the list.
I'm now talking to some grid owners and they are saying there has been a mass exodus from SL.