Does this sound ridiculous? It should. It is equally ridiculous that users of one social network can't interact with users on another. This is the problem that needs to be fixed, not ads.
In an Internet 1.0 world "Facebook", would have happened via a 'social' protocol on the Internet just like SMTP, and Zuckerberg would have been a hacker icon just like a Tim Berners Lee.
And I went on to write:
PS: If mails had happened in Internet 2.0 world. We would have gmail users only mailing to other gmail users and hotmail users only mailing to other hotmail users ... but actually gmail would not have happened as Sabeer Bhatia or Microsoft would have been still ruling the mail world ;-) ... But thankfully it did not turn out that way, as there was already a protocol called SMTP!
The comment was this one: http://news.ycombinator.com/item?id=3949048
Edit: Added more relevant text