I wouldn't be able to go there, personally.

If it's a difficult conversation, maybe you could maybe start with... "I've been getting compulsions to check your facebook page. How would you feel about that?" and see what the reaction is. Then perhaps go a little deeper later on?

Openness is definitely better than not, but once you cross the line into intruding into their personal life, all bets are off the table regarding what the reaction will be.