While I agree that Twitter / Yelp should be scared (very...very scared,) you forget that Google makes little to no money on all of those things you mention. They are a search company. Facebook stands to be a giant threat to their search business.
Per Alexa, the average FB user spends 30+ mins on FB each day; the average GOOG user spends <10 mins on GOOG. And FB has far more extensive info about each user.
I think the risks to GOOG are generally underestimated. It's going to be a great stock to short within the next 3 years.
But then what isn't?