I think this is a very interesting idea and things like a 'group' or an 'event would simply have their own private/public key to allow subscribers etc. But two thoughts spring to mind:<p>1). Without the marketing/advertising opportunity, who will be prepared to pay for a facebook like IT infrastructure?<p>2). In some ways I quite like that criminal or otherwise sinistre behaviour/communications could potentially be intercepted and policed by some authority for the benefit of others - ie, the bad guys dont have anywhere to hide... (I'm expecting an onslought of critism from this comment!)