How To Grow Userbase Effectively?

I'm assuming you are getting tired of your "pre-launch" promotion activities to drum up beta users. At this point, I would suggest: Just Launch It. Get it live, have a great intro into subscriptions, contact bloggers, send press releases, cross-post positive and negative posts on related forums, etc.

Getting on Mashable and some of the larger blogs will get some major traffic and lead to potential coverage elsewhere.

Just keep doing what you are doing. I don't know you and I read your blogpost and signed up for the preview. Make sure you talk about it and don't be shy about asking people to look at your app.

Good luck!

Well I signed up, thought it looked interested after looking at the tour, but when I signed up it just said "DAMN YOU". That might be why you're not getting new users :)