How do people know I'm online on Facebook? I'm not even on Facebook chat.?

[Doobop]

I just posted a comment on a friend's picture, and 10 seconds later I get a message on the inbox. It's annoying.

[Cjmjm2]

because it shows up on their newsfeed whenever you comment on something. If you comment on a group page it shows up in all of your friend's newsfeeds. If you post on friend's page it shows up on the newsfeeds of people who are mutual friends. You can edit the settings for that in your privacy settings

[Clairecross Broad]

yes it will shows in your live news feed
you edit it in your privacy settings...