Carry them over from another product you've made in the past. I work for Appfluence, and we make Priority Matrix, and app for managers. Every time we build a new version (for a different platform), it's easier to get traffic, because some of the people that were using the app on the original platforms migrate to the new ones.

I realize this doesn't apply to everyone, but if you can use it, it's great. Zynga is the master of this, but network effects work at all levels. Even if you don't have another product, think of what other social circles you can tap into. Chances are you talk to potential early adopters on a regular basis.