So this is rather simple, 2 different buttons at the top for either

a) someone posted in a thread you are subscribed to
b) private message

Also, the (Number) notification you get on twitter/facebook would be useful, as there is no way of telling when someone's messaged you unless you check the forums, so I can go for hours without replying to people it could look like

| (1) Toribash Community |
Feedback or any other suggestions are encouraged

Thanks

Thread tools > subscribe to thread.