Forums are social media, especially so for sites like Reddit and Lemmy where the subforums are community-created.

Wikipedia:

Social media are interactive technologies that facilitate the creation, sharing and aggregation of content (such as ideas, interests, and other forms of expression) amongst virtual communities and networks.

Merriam-Webster:

forms of electronic communication (such as websites for social networking and microblogging) through which users create online communities to share information, ideas, personal messages, and other content (such as videos)

Britannica:

forms of electronic communication (such as Web sites) through which people create online communities to share information, ideas, personal messages, etc.