A couple of things that have been effective for me:

1) Block said online service using the hostfile until addiction becomes manageable / goes away. Repeat as needed.

2) For work, keep a notebook. Review and set goals at the start of each day or week. Review and update notes / goals at the end of each day and week. Your own agenda, your game to keep yourself motivated.

Now, since your addiction is blocked and you have something to do, you'll at least get it done before turning to your addiction to kill time / unwind.