Reading the above posts I can't tell if this has been answered or not.....
1) Only people you have had a "chat" with can see that you are online.
2) If you click on "contacts" and find someone you haven't had a "chat" with, select their profile pic and select the i for information, it'll tell you the last time they were online.
3) you can do No 2 even if they don't have you as a contact. For example, I have my boss as a contact and I know for a fact she doesn't have my mobile number in her phone, when I click on her profile I can see that she was last active last night (15/06/2015)
Hope this helps
