Did they get w2s and 1099s? Did FB did employment authorization verification? Did fb verify that the contracts were in fact signed by authorized guardians? A minor signing a contract has no meaning - so no “employment” contract would be valid.
Facebook was paying 13 year olds to use the app, so that argument isn't really available to them unless they want some pretty significant civil penalties straight from the US Government. (I strongly doubt it'd fly either way, but "we hired underage contractors" really won't.)