[sorbus]

If you notice it when you're still on the page, it's easy. However, as I understand it one would have to go to their facebook page to see this, and it seems unlikely that most users would be doing that (constantly watching their facebook page, not just the homepage feed). If someone is "liking" a lot of pages, then there's also the difficulty of figuring out which pages the spam is coming from - even if it would be possibly to determine through a process of elimination, users would have to remember every page they liked by the time they noticed the spam.

Not a big issue now, but if lots of pages start using this capability, it could become a problem, albeit of a very minor variety.