In social media context, AMA is an acronym for "ask me anything".

AMAs are a type of informal interview where every member of a social media group or community is welcomed to participate. The questions are generally left in public comment sections, where interviewee can easily respond to them.

Reddit has a whole subreddit dedicated to this type of content — r/IAmA, but you can encounter AMAs on other social media platforms too (e.g. Twitter).