[stonemetal]

They could host it behind age checks. It seems like it would be easy enough for them to perform automatically.

[nitrogen]

They could, but then they'd lose a large chunk of users who don't want to be even remotely associated with such a site.

[william42]

And this doesn't apply to Holocaust deniers because...

[Anderkent]

Because people don't care about Holocaust deniers as much as they care about 'indecent' pictures.

[potatolicious]

In fact, Facebook's age-check would be 10,000,000% more effective than every other age-checked page on the internet.

coughSteamcough