Ask HN: How to market a new social network?

I've built a small social networking service that is a spin on Twitter, amongst other services. The main problem I'm running into is that it's hard to get users to start using it.

I think most people find social networks interesting because people they know are on it. But without an established user base, how do you actually attract the early users?

I've tried the general spray and pray method of submitting to various blogs that cover startups and social media, at various tiers. I've also reached out to friends to get them to try it out, and a small contingent have been using it and do enjoy the service. However, it seems hard for me and them to spread it beyond that.

Social networks are all about virality. I've added various triggers for my service to piggyback off of Facebook and Twitter, and allow the users to share/publish to their friends. However, I need that initial group of users in order to get the ball rolling. Any ideas on the most effective ways to do that?