Since facebook is deprecating fbml, could someone point me in the direction of what would be the replacement for the multi-friend-selector? We use this to allow facebook users to send invitations to their friends to let them know about our site.
You might want to have a look at Facebook's Send Dialog and Request Dialog.
There also exist two JQuery Plugins you might be interested in. The first is an autocomplete plugin which includes a Facebook theme and the second is a full friend selector plugin. I already used both on production:
http://loopj.com/jquery-tokeninput/demo.html (see the facebook theme)
http://labs.thesedays.com/blog/2011/06/20/the-missing-facebook-interface-component-for-friend-selection/