Why don't you "motivate" frequent users to start paying?

ex. by making the free version (gradually) worse until you see revenue.

ex.

- non pro user pastebins last 5mins, 5 views etc

- non pro users can post 1 pastebin per day (cookies, IP)

- non pro user pastebins are max x characters long.

- you get the idea

- there are clever people on this forum who will come up with good ideas to help you out.

- do everything BUT show ads, PLEASE. Internet's sole purpose is almost all that already.