Consider that they're hard at work at this website as well. Remember that much of what they do is designed to corrupt our political system. To the extent that it involves spending money to influence our elections, it is illegal.
From a Washington Post article on the subject:
"The law is clear that foreign nationals and foreign corporations are prohibited from making contributions or spending money to influence a federal, state or local election in the United States. The ban includes independent expenditures made in connection with an election.
But whether the Russian company broke the law by running ads on Facebook comes down to two big questions: What was in the content of the ads, and did a U.S. campaign assist the company in placing the ads."