But they had three years to think about monetization, and the BEST plan they came up with was advertising. Not freemium, or selling products, or any other business model.
Advertising.
TV advertising "started out slow" too. Now it's 1/3 of the viewing experience. With ads, you never, ever, get fewer ads. It's always more. More annoying, more intrusive, on a good day just more "targeted."
Of course, we've seen ads coming to instagram for years. The good people at Facebook somehow managed to reinvent the way everyone interacts, but can't find a way to make money directly from the people who use their service.
TL;DR...Monetizing isn't bad. But advertising is unimaginative.