Spam volumes are also down dramatically due to conventional means like making it illegal, and taking down bot farms that were sending it. Also the introduction of DKIM and DMARC making it harder across the board to be seen as a legitimate sender, and the fact that many-to-many emails are a huge red flag for spam filters, and not a concept for social media.

Spam is a dramatically easier problem, and has many more mechanism to suppress it both legally and technologically.