When eBay sold Skype (the first time), a lot of the commentary was along the lines of 'of course they never integrated it - get two people talking and the first thing they will do is agree to sell at a reduced price to avoid the eBay commission'.

So when the OP says "It makes us wonder why the startup didn’t release the feature sooner" my immediate thought is: if I knew a friend of a friend had a spare room available, I'd be communicating through Facebook to arrange it for cheaper (or, if they're good enough friends, free) rather than through Airbnb.

Now, the connection for Airbnb isn't as direct as with eBay - I would need to go through the friend, and that adds time and drama. Maybe the Airbnb team are confident that that extra hassle will outweigh any sly users of the system; or maybe they're just super confident that adding social validation to their potentially risky business [1] will drive customers in droves, so a few sly users really don't matter [2].

[1] Potentially risky in that paying to sleep in someone's spare room (or even a private apartment) is a riskier travel proposition than paying to stay at the Hilton, or even a reputable Hostel.

[2] And, to be fair, they're probably the users you don't want anyway.