Although 5 years old, /r/tumblrinaction didn't really start being used until 4 years ago.

For non-reddit users, it is a reactionary subreddit for crazy Tumblr nonsense that floats around the echo chambers of the site.

I think the decline in use was caused in part by the polarization of extreme feminism (think: all men are rapists) and red pill (think: feminism is cancer) nonsense. It became hard to have any reasonable conversation without posts being shared and brigaded by extremists.

Other platforms became better, IMHO.