The Pittsburgh shooter allegedly had a following on a social network that's trying to be the far-right's alternative to Twitter - here's everything we know about the site

Gab was launched in August 2016.

Initially, consumers could only use the site if they'd been invited to register.

The site was cofounded by Andrew Torba.

Torba previously cofounded AutomateAds.com, previously known as Kuhcoon, a social media advertising company that had been backed by Y Combinator. Rival AdHawk purchased AutomateAds last year.

Torba founded Gab with Ekrem Büyükkaya, with whom he'd worked at AutomateAds.com.

Torba launched Gab as an alternative to Facebook and Twitter.

Torba told BuzzFeed News he had been frustrated with the way "left leaning Big Social" sites were filtering posts, feeling like they weren't qualified to judge what was news or harassment.

"It didn't feel right to me, and I wanted to change it, and give people something that would be fair and just," he told BuzzFeed News.

Torba touts Gab for its commitment to free speech.

Gab's guidelines prohibit users from posting certain kinds of things, such as threats of violence, illegal pornography, and other users' private information, without their consent. But it generally doesn't bar posts that many might consider abusive or hateful. Instead, it offers features that allow users to filter out offensive posts.

The site quickly became a popular hangout for white supremacists.

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.

The service is relatively small.

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.

By comparison, Twitter has some 326 million monthly active users, including 67 million in the US, and Facebook has some 2.2 billion users a month around the world, including 185 million in the US and Canada.

Gab's run into trouble before.

In August, Microsoft threatened to kick Gab off of its Azure cloud computing service for hosting posts that advocated genocide against Jews. The author of the posts later deleted them.

Gab ran into a similar problem last year, when its domain registrar, responding to anti-Semitic threats, threatened to seize its domain if it didn't find a new host.