I've added the FB Username as something to look into. The fb_12345 name is there as a legacy option, since that was the original (only) method we used to create usernames. Now you can select first.last and firlas as automatic usernames as well in the configuration area, which gives the flexibility that most users (we think) are looking for. If you find that too restricting though, definitely let us know.
The main problems I can see that we'd have to work around for using the FB Username is:
1) Yes, they are unique on Facebook. However, that doesn't mean that someone hasn't already used that username on the Joomla site. We'll need to have a fallback (add a 123 to the end, etc)
2) Not everyone on Facebook has set their username. In that case, we'll need to have a fallback to one of the other auto-username schemes.
Everything's more complex when you get into it.. but we do like the idea. Won't be something implemented in a minor (4.2.x) release, but will look into for the 4.3 release, if possible.
Thanks for the suggestion,
Alex
Suggestion: Joomla username = FB username
