Facebook.com, pinterest.com, instagram.com, linkedin.com... All the giants have .com domains in the social media sphere, so is it important to have one as well if the business will be in the social media sphere?
I upvoted Caroline's answer because it's the most direct answer to your question. Joseph's perspective is that of a domain-owner and although thoughtful and nuanced for a very general question, I vehemently disagree with the assertion that "consumers expect .COM in America." I think that might be true for eCommerce, but there's plenty of other ways to build trust than just the domain itself.
Oscar's passionate assertion of the value of the .COM is just not something I would give credence to. It's absolutely a nice to have and if your business scales, not owning it early will only drive up the price later. That said, I'd rather validate the core offering first, and then invest in buying the .COM before massive traction but ahead of a lot of traffic and publicity.
I generally start with .co as the preferred domain when the .COM isn't available.