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)