I worked on this exact same problem one summer, and it's a much harder problem than I anticipated. The makers of this would probably concur -- I haven't gotten great results, even using their suggested searches (e.g. "entrepreneur brings up Demi Lovato, Tiger Woods, and Pitbull).

There are both tweet-level and account-level signals. But some of those signals can be artificially inflated -- follower counts, favorites, etc. One of the highest signal features I focused on was whether or not you were RT'd or @'d by another influential account. Of course, this is a somewhat-recursive problem which made it difficult to wrangle with.

I eventually got something to work, but it was only possible because I had access to the Twitter firehouse and through the manual weighting of features.