There's a Silicon Valley mentality about marketing that's always bugged me. It says that you should never have to market your product and that you must acquire your users for free to insure success. This myth was created by the viral sensations like Facebook and Twitter, or sites that came before, where user acquisition is free, and the product seems to scale infinitely on its own. These are just plain 1 in 1,000,000.
To solve the problem, as the CEO of RingCentral Vlad Shmunis puts it, "some of us have to work for a living." RingCentral seems to be everywhere. Outdoor billboards, radio ads, event sponsorships (thanks!). They figured out very early on that they had a product market fit and that they needed to tell people about it.
They were fortunate to be one of the first major AdSense partners and were able to ride that wave. Shmunis points out a critical component to marketing in that there little long term product success without great word of mouth support. But entrepreneurs should be supplementing that with advertising right from the beginning. See his full comments about it above.