The last seen rules apply to your online status as well. People can only see you online if you’re sharing your last seen status with them.
There are some exceptions because sometimes it is obvious that you are online. Regardless of the last seen settings, people will see you online for a brief period (~30 seconds) if you do the following:
- Send them a message in a one-on-one chat or in a group where you both are members.
- Read a message they sent you in a one-on-one chat.
- Broadcast a “typing…” status to their chat with you or to a group where you both are members.
If you’re not sharing your last seen timestamp with someone and don’t do anything of the above, they’ll never see you online. Another way of achieving this is to block that person.