The same exponential social forces that drove Facebook to prominence can turn on it surprisingly quickly and cause exponential decay. If anything keeps them up at night, this is it.

I've refused to join Facebook because of an inability to partition my life appropriately. I have at least three personas I don't want to just run together (family, friends, coworkers, probably an online shard as well). If people as a whole realize they can't express themselves freely with their friends because mom is online, that's going to be a big problem for Facebook. This could yet kill them if they continue to choose not to allow fragmentation.

Also, much has been made of how this young generation has no desire for privacy and can't understand why anyone would want it. I've thought for a while that there is still the plausible outcome where instead of never growing a privacy sense and being unprecedentedly open, that they would instead collectively discover the ancient social reasons for privacy the hard way instead. It will never be a single event that makes good copy, but it sounds to me like we may be down this path now, though still in the early phases.