It's not just the prospect that FB or Twitter figure out a new wildly-profitable promotional model themselves, but the prospect that someone else figures it out

It's not just the prospect that FB or Twitter figure out a new wildly-profitable promotional model themselves, but the prospect that someone else figures it out, and FB or Twitter's large audience/social-net/technology defensible asset is still the best way to deploy it. Compare: Goto.com (Overture) pioneered pay-per-click auctions, and were already a profitable public company even before Google adopted a similar model. But only combined with Google's natural-search advantages did that model become a profit supervolcano.