Gab, which bills itself as the free-speech alternative to Facebook and Twitter, has become a haven for far-right extremists. The site does not police hate speech, instead encouraging users to take advantage of its tools to filter out posts they find offensive.

Gab has an open-door policy for far-right figures like Milo Yiannopoulos and Alex Jones, who have been kicked off of Twitter and other tech platforms.

Although the site purports to be open to anyone who shares "in the common ideals of Western values, individual liberty, and the free exchange and flow of information," it's most frequently populated by far-right ideologies.

As of the beginning of September, Gab had 635,000 registered users, according to a document it filed with the Securities and Exchange Commission. That's up from just 394,000 in March, but it's a drop in the bucket compared to the social-media giants.