

I like the idea of only accepting Facebook registrations (even prefer it over Twitter accounts).
You may want to try adding non-latin characters used by foreign languages to your spam trigger lists as a way to prevent those posts. For example characters, look through the /languages/translit.txt file.
The Facebook Module for Pligg CMS!
Register, Login, and Submit Stories with Facebook. An absolute MUST HAVE for all Pligg sites!