Foursquare, Pinterest, and Tumblr have plenty of users; now they need to make money. Here's how they plan to do it--and why it may be a challenge.One of the common paths high-profile tech entrepreneurs have taken in the last seven or eight years goes like this: build up the user base for your Internet service, and eventually you'll find a way to monetize the traffic and build a sustainable business.Hey, if the likes of Facebook and Box did it, so everyone can, right?Not necessarily. It's become painfully obvious at this point that moving from obtaining users to making money has not become ... Read More