I'm surprised they are so creepy looking. You'd think Facebook could afford a decent illustrator to mock up the avatars? But knowing Facebook they're probably algorithmically generated avatar images...
Uncanny valley is not the explanation for all creepy looking models. It is specifically a problem with very realistic looking models and textures not being quite right, for a variety of reasons. These models are not attempting to be realistic, nor are they particularly close.
Not much creativity plus low-poly gives you that creepy look.
They can't be particularly creative without alienating subsets of the audience. What you're seeing is basically committee-approved art: many layers of approval given the ability to veto stuff that's 'weird', leading to a bland pablum that offends everybody equally.