I think the end of the Bush administration (as well as the departure from the political scene of issues like the Federal Marriage Amendment, Intelligent Design etc) had a lot to do with it: atheists became less united as a force, and were split by the less explicitly religious social issues that began to domaite media in the Obama and Trump administrations. And seeing as we're talking about the quality of online debate, the impact of gamergate was quite dramatic in how it created a fissure within the "sceptical community". A fair amount of online figures heavily invested in social issues on both sides grew out of the community, including figures as disparate as trans advocate ContraPoints and UKIP-stan Pat Condell.
For all the hand-writing of atheists as being fedora wearing neckbeards I don't see the community as being different from what it was at the end of the Bush administration. Secular voters are united on voting against the Republican Party that is still very religious in nature. It's not the political party that resembles a conservative atheist like George Will but the Mike Pence's of the country who seek to end freedom of religious expression. I think the mainstreaming of secularism has played a part in the downgrading of online communities, as it isn't stigmatizing to come out as a non-believer like it was in the Bush years. While neckbeard dwellers want to compelled their lack of belief as being special it just doesn't carry the same rebellious nature it did when the internet was a place for outsiders to have a safe spot.
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