Facebook and YouTube have a lot of user who spend a lot of time there. This at least makes an ad model possible. Twitter does not have that, especially consider

Facebook and YouTube have a lot of user who spend a lot of time there. This at least makes an ad model possible. Twitter does not have that, especially considering how long it's been around (it was launched about 6 months after YouTube) and how unbelievably overhyped it is, and a large share of the users it does have are spam accounts. Only time will tell, but I think along with "make their investors infinitely rich in the not too distant future" there's also a strong option of "completely forgotten in the not too distant future".