I'd suppose the answer would generally be yes depending on who you're talking to but I'm a bit unsure.
Just wondering if it isn't weird to speak very casually online
for business (or what would normally be politely addressed eg to an estate agent, japanese teacher) emails, absolutely.
More 'anonymous' stuff like 2 chan, youtube comments etc you can be more casual.
But if its an app where youre chatting with someone you dont know well (eg language exchange, dating app etc) its worth being polite and maybe using keigo
If you are using a social network tool, probably you don't have to worry about the words so much. Other than that, It's possibly better for you to use keigo in order to be understood by other people.