Having some optional for-pay features seems like the no-brainer way for almost of these "give service away for free, get huge if popular" web businesses to get revenue. It should work for NYT, Facebook, Reddit, Twitter, etc. Essentially, freemium bolted on late. Even if not all users would be willing to buy anything, there will always be some folks that do. Find out what those folks want and are willing to pay, and make that happen. Done.