Tumblr reblog
Tumblr's sharing mechanism: reblog a post and add your own text/comment to a growing chain. Unlike a retweet, the entire comment chain is preserved — a post can accumulate years of additions from different users.
Culturally unique: a post can "die and resurrect" multiple times as it gets reblogged into new circles; comment chains become works of art/jokes of their own. The foundation of Tumblr's fandom and meme culture. Distinct from a retweet (no chain) and a quote-tweet (one level). Likes on Tumblr spread nothing — only reblogs give reach, which shapes behavior ("don't just like, reblog!"). Tumblr (2007) is owned today by Automattic (WordPress). Many meme formats and expressions spread from Tumblr reblog chains to the rest of the internet.