After reading these two documents I still have more questions than answers.
1. Do I need to ask Twitter/Facebook for permission?
2. Do I need to register anything anywhere? There are those "developer sections" on Twitter/Facebook, do I need to mess with them?
3. Can I use custom made buttons or maybe I have to use the buttons they provide? According to the links above, they may have some objections about custom design of the buttons. On the other hand there is so many apps and sites with custom buttons so I probably misunderstood something.