I just opened it today to see what's up and was greeted with a page-wide popup asking me if I was a child or an adult. I'm not sure what's going on over there at this point.
Maybe block access to mature audience pages? Or disallow address to social features to minors? Or as the other user said, advertisements?
I know my state recently passed a law that requires identification of SM users to prevent exploitation of minors (not gonna work, but that's the law), so maybe they're trying to avoid lawsuits/shutdowns.