Facebook or not facebook, are you seriously think that if you will clone design, functionality of facebook - peoples will happy to join and join to similar communities? I not sure. Easy website should have own zest (idea). No need clone it again and again :-)

As core suggestions, can offer next:
* how about CSS3 + HTML5? Most of browsers support it and it giving really interesting results. IE6 soon will die in any case (by words of MS)
* implementing (to dolphin forms) HTML5 validation (at client see more side) ? .. or, maybe jquery validation (so we don`t will need to submit data to server for validation)
* how about implementing (or integration) of new captcha (as example math-based or another ready) ?
* how about making less CSS files? And keep several CSS files instead tens (for disabled CSS caching)
* same for JS files
* what about making builder of pages more universal? As example - allow which design box will used for boxes. Plus, make available blocks not only for member/guests, but also for memberships too.

it possible to continue adding suggestions