Congrats on getting traction!
Dave McClure's Startup Metrics is awesome, though overnight success usually takes many years of hardwork.
The resources I would like to add are the following -
- Crossing the Chasm - Geoffrey Moore - This book is said to be the marketing bible at Microsoft back in the in 90s (and probably still is). One of the key insight in this book and series is that once a software starts achieving traction, you'll need to find pockets of community users that fit the use of Tweetminer. You'll need solve all the potential adoption issues (ie signup).
Are you able to find out more from the free users on the features that they would be willing to pay for? Is there another way to monetize their traffic like the way Mint did? Or perhaps have the free users helping promote the usage of Tweetminer?
The Tipping Point - Malcolm Gladwell - The book discusses how epidemics are spread - and in hindsight of viral adoptions of Hotmail and Skype - a great read and if you can identify amongst your users who are the mavens - and then proceed to look for more of them in the market.
Clickbank - I see that you have an affiliate page, Clickbank traffic may be appropriate for your service.
Another thing to note, there is no link to pricing information on your site currently - while I am guessing that you are looking to convert folks to use it after they have signed up - the disadvantage is that companies that would be keen to resell this service, or recommend it to their clients, may have difficulty finding out what the cost is.
Your service seems prime for media agencies and and their clients to use.
In summary, both Crossing the Chasm and Tipping Point talks a lot about community adoption, which in momentum drives up the usage - an interesting observation of this growth is stackoverflow.com - which when they first started out, focused on software developers. Stackoverflow recently moved into high gear with penetration into different segments of their own, such as superuser.com and serverfault.com. In fact, this site is an extension of their model where the software is now used by communities for startups, parents, gamers and a lot more.
The key in the first step is to saturate a niche fully well - and thus in your case is to look at your most active users - and look to identify who and where else can you find more clients like them.
All the best!