In many cases it's true that an S-Corp or LLC will be preferable for a business, because they can be treated as pass through entities (under the IRS check the box regulations) and not subject to federal income tax at the corporate level. But S-Corp's have some significant limitations for a startup company seeking investment--most importantly, they cannot have multiple classes of stock.

As others have mentioned, we've started with the C-Corp form because we think it will be what most of our users would prefer initially, but we'll look at expanding it to other forms.