From people I've observed, I would say that at least half are attempting to subdue the anxiety that they are somehow missing out on something that is going on. As if they couldn't be the 10th person to accept a party invite, or couldn't bear to not be the first to know that somebody is pregnant or getting married or breaking up, or whatever. I think the OCD comparison is accurate. Not for everyone, but for a substantial subset.
Repetitive Facebook checking is supposedly more like an addiction - people are chasing the endorphin rush that comes with finding a new message (or winning at the table) rather than trying to chase away anxiety.