The second Eternal September, directly related to the rise of social media, which I think was ultimately fueled by the mass adoption of smartphones.

Before, the Internet was mostly inhabited by people who wanted to do something specific, and were at least minimally invested - they had to actively use a computer to access things on-line. Nowadays, the Internet is inhabited mostly by people ready to judge you while sitting on a bus, because their phone buzzed them a notification that their friend retweeted a story about some stranger being outraged that some other stranger retweeted a link to your SE question.