I'm wrapping up my pre-launch phase and I found it most effective to make personal, one-on-one, connections with my (really) early adopters (mostly friends and family).
I used general emails with users on BCC and social media posts, but they only hooked a very small percentage.
One-on-one emails and SMS/Chat/FB messaging friends was most effective. Sure it can get to be a lot of data/connections to manage, but, when doing customer development it was the best tool I found. Of course, in person is best for customer development.
Interested in what others have found effective also :)