I tend to end up posting on a lot of Atheist FaceBook sites, not because I am an Atheist, but because I tend to agree with them more frequently than not. I am in a really strange and unique place in the theology department...
Atheist Republic tends to have people who are very politically minded, and in my experience that means they are going to be more abrasive in their discourse. Sadly, the vast majority of people (anywhere, but especially on social media platforms) are not sensitive to disability unless it impacts them in a major way. Saying, "I have a friend who..." or "I know somebody that is..." is a bad indication of being sensitive to these issues anyway, but Autism is in a class on it's own because of the way it presents, so people tend to be very bad at misconceptions.
And Atheist pages are not known to be kind to strangers. I get a lot of flak for my views on religion on these sites, but I also tend to ignore the insults and stick to the facts. I learned throughout my life that people find it harder to argue against facts, and so keeping my focus on what is relevant to the facts tends to get me by in discourse. It does tend to make me seem a bit bookish, nerdy, or stilted in encyclopedic info dump, but I have been called worse.
We Freakin Love Atheism seems to be a bit more mellow, but if you are looking for a page of likeminded Atheists to follow, David G. McAfee is one of the more approachable pages to follow on FaceBook. The Plus side is he claims to be on the spectrum himself, so there is some sensitivity there. Doesn't mean every Atheist you'll find there is sensitive to mental disability, but you are more likely to find people who are.