CNET brought up the point this morning, and a look at the site's TOS reveals that the (sometimes) flattering pics may not be legally cool:
So unless people have been trolling Creative Commons for snaps, it's likely that they're violating Facebook's rules.
As far as we know, the site hasn't taken any action to quash this trend (Full Disclosure: As you can see above, I gleefully took part in it as well). Of course, using a picture of celeb in lieu of your own image is nothing new. In fact, using unlicensed images is really nothing new -- blogs, Twitter accounts and social media site users do it all the time. The sheer magnitude of people taking part in this meme -- along with the high visibility of the TOS violation -- is what's noteworthy. Still, we really doubt that any measures will be introduced to put an end to the trend, which is only encouraging more interaction with the social networking site.