It's not just Flynn. It's a large number of people in his campaign and others currently on his staff. Then there's this: 
In congressional testimony, the acting F.B.I. director, Andrew G. McCabe, has confirmed the existence of a “highly significant” investigation into possible collusion between Mr. Trump’s associates and Russian operatives to sway the presidential election. The pace of the investigations has intensified in recent weeks, with a veteran espionage prosecutor, Brandon Van Grack, now leading a grand jury inquiry in Northern Virginia that is scrutinizing Mr. Flynn’s foreign lobbying and has begun issuing subpoenas to businesses that worked with Mr. Flynn and his associates.

That's Congressional testimony for all those who want to use the "Fake News" smokescreen.