In a sensible and effective post about Google+, Olivier Blanchard captures a basic challenge for Facebook:
Facebook’s functionality is still very limited. It doesn’t really plug into productivity and collaboration tools, and this is a problem as users (consumers) increasingly look for seamless integration of word processors, email, video conferencing, VOIP, calendars, mobility, spreadsheets with their social platforms. The simplicity of Facebook’s design and the limited amount of customizability that helped it compete against MySpace (and win) may also bring about its own undoing now that digital platforms have matured.
I highly recommend reading the rest of the post here.
Keep this in mind: Facebook is to AOL as Google is to Google.