Hello, welcome to Meta! 
I don’t know much about Facebook. Let’s see:
- Topic: This is the subject of a discussion thread. It contains posts.
- Post: A post is any contribution made to a discussion. It can be the initial message.
- Reply: A post directly addresses an earlier post within the topic or to continue the discussion.
A post, comment, and reply are essentially the same: messages that contribute to the discussion.
The term comment is not often used here. However, it’s used in the Discourse Post Voting plugin, which represents the tiny replies under the vote. (Thanks to Jammy for letting me know about it)
Does that clarify?