I think they are similar to FedEx. FedEx knows that millions of packages per day are transporting illegal goods, any bad enough accident shows it. However, FedEx would absolutely go bankrupt if they tried to open every package and make sure the contents were good. At the end of the day, that's the government's job.

If the DEA and ATF wants to staff every shipping hub with people checking every package, that's fine by them (though admittedly it would hurt revenues).

For Google and Facebook and all the other user-content sites, it's just impossible to actually, fully uphold the law themselves, so their best bet is just to try to make it a pleasant experience for the users and leave upholding the law to the upholders of the law.