Another approach is to make it backward compatible with something else. For example, if you wanted to make a new site/network similar to StackExchange (but presumably with some revolutionary changes; not just a "me too" copy), you could start by adding an interface layer using the StackExchange API and adding some of your new smarts to give your users some of the benefit of your tool, on top of the StackExchange content. Most likely you wouldn't be able to add all the new goodness there though, so you would also do as other comments have suggested to build up your own content. Maybe even incentivize users to add stuff based on StackExchange answers they find (assuming some manual intervention is necessary to make the answers suitable for your site - and also assuming you comply with terms of use and such).
It's hard to give exact examples without knowing what you plan to do of course, but that's the general idea. Make it backwards compatible with whatever the closest thing is out there already, so that even in cases where you don't have any of your own content to return for a user, you at least have something. Then do everything you can to ramp up your own content asap.