I can tell this one is so much bigger than most social network sign-up marathons. Along with the huge amount of people joining, I'm also seeing friend requests from people I haven't talked to in years, some over a decade or more.
Also what's most interesting is that it seems to be evenly split between my Facebook friends and my Twitter friends. Normally it's just the Twitter friends who are early adopters when the latest and greatest comes out. So that's promising.