Minitel adoption was never as big as with the current Internet, and users didn't suddenly migrate from one to the other. Minitel mostly bled to death as the Internet grew in popularity.
Minitel killer app was the yellow pages.
The other predominant usages were mail order and sex chats/porn. Some games too but nothing significant and nothing social, at scale at least.
Was there a particular social forum that ruled minitel - did it transition to the web?