Though online appointment-making service ZocDoc raised $3 million in its first round of funding from Khosla Ventures earlier this year, two new investors have jumped on board, giving the startup an undisclosed infusion of venture capital. One is Amazon CEO Jeff Bezos, contributing via his personal investment company Bezos Expeditions; the other is Marc Benioff, Salesforce.com founder and CEO.

The initial round from Khosla allowed ZocDoc to roll out a few new features in recent weeks, most of which will help scale the service across cities and practices. What began as an online appointment-making tool for dentists now supports ophthalmologists, orthopedists and Ear, Nose and Throat specialists. Even more specialties, like gynecology, will be added to the site in coming weeks.

No specific details have emerged regarding Bezos’ and Benioff’s plans for ZocDoc, though both individuals have more than a little experience launching successful Internet companies. Yet having high profile backers doesn’t always spell success in the long term. ChaCha, the web and mobile search engine that received support from Bezos early last year, recently downsized its offerings as well as its staff.

Given the current economy, investing in an idea that is still relatively new in the healthcare industry could be risky, even for someone like Bezos. But before I even got a chance to ask, ZocDoc co-founder Trevor Leb assured me that his startup has plans to weather the economic storm. With a crunch in spending across all businesses, he says, doctors may actually be more inclined to turn to a service like ZocDoc to reach potential patients, target their marketing strategies and establish an online presence.

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