An observation: I find that when I'm posting on forum-like mediums, I always capitalize sentences properly, and end them with a period when appropriate.
When I'm posting on chat-like mediums, I only capitalize proper nouns and "I" at the beginning of a sentence. I also don't use periods (question and exclamation marks are still used).
The Fediverse feels right between the two in terms of conventions to me. Sometimes I feel chatty and drop capitalization and periods, sometimes not.