I still take issue with the idea of "oh we'll worry about making money later" If facebook is the prime example then just take a look at what a piece of crap its become for users. Facebook couldn't find a decent revenue model so now they're cannibalising their users by making the experience terrible. Which in my opinion, long term is going to be their downfall.

Also, getting lots of users and then just slapping ads on your site is also a terrible business model and its what you tend to do if you haven't bothered to think it through clearly before starting.