Rapportive is one great tool that's iterating pretty quickly! Looks like someone is putting the VC/angel money to great use. It's the one reason that I don't use Sparrow instead of GMail. However, the main question again: how will they earn money?
The plan is to go freemium: build an amazingly useful product, and charge money for premium features. I know that startups very commonly say that this is their plan. We've thought very hard about what makes freemium work: http://blog.rapportive.com/is-freemium-right-for-you . We're pretty certain that Rapportive has all the right characteristics.
One quick question: the upgrades that we see today (Facebook comments and the like) - will they always be free or will that be a part of the freemium suite at a later date?