I'm not sure if it would be even possible to collect analytics on this, but I think a real danger to Facebook could be that while people may not leave the site,

I'm not sure if it would be even possible to collect analytics on this, but I think a real danger to Facebook could be that while people may not leave the site, the confusion over what shared information is shared with whom might cause users to use the site less and share less on it. As an example, most of the non-technical people I know, when prompted a few weeks ago to replace their 'About Me' page with a series of their fan pages, simply opted to make that page mostly blank. Even though these people are not leaving Facebook, this still hurts them because their value is not only in how many people are on there, but what those people are actually doing and sharing. It's not just the nodes that make a network, but the connections between them.