No, but there are several laws that could enforce disclosure of such activity, especially at the border. If a trivial automated web search can prove you failed to disclose anything on your entry papers, then you can forget your visa waiver (and likely find yourself on a nice database of people to monitor). Obviously they say disclosure is “optional” now, but that’s just one word that can be dropped the first time some random terrorist is found to have engaged in particularly stupid social-network activity.
If there is one thing people with “inner-big-brother” know very well, is how to turn anyone into a criminal.
