There's a section on it in the online visa application form. They make clear that if you omit items (eg you share you facebook page but not your twitter) and they find out then that may be grounds to deny your visa application.

The entire form starts with copy that goes something like "by filling out this form you waive any right to privacy". It's extremely aggressive. The US hates foreigners.

(source: my first and only trip to the US some 3 years ago)

It is part of "random" checks. There have been cases of people being denied entry for unlocking burner phones or social media, and -allegedly- people have been denied, because the TSA didn't believe them they weren't using social media.

Furthermore, it isn't only illegal things thst could get you singled out. If I remember correctly, a simple facebook post accusing the US to be war criminals and the greatest threat to peace was used to deny someone entry.

(On mobile atm. will update with sources in 4 hours)