I'm not an avid Twitter user and this is only my personal impression rather than a guess why anybody else might be abandoning your app. That said:
Initially the idea of filtered tweets seemed compelling. I think everyone has that one spammy friend who they only follow because unfollowing would be impolite. But soon after signing up I realized that it's simply too much work to be worth it: not your app specifically, but using a separate client and having to come up with filters. Filtering and organizing is simply not natural for Twitter - it's meant to be a continuous stream where missing something is no big deal. I could possibly use filtering if it was in a form of a browser plugin (so I could still use twitter.com) and had just one option - to suppress tweets from a specific account.
Unforunately, as we're a platform and not an addon we have to provide a separate interface for the app.
In reply to Twitter being a continuous stream, that is how some people (yourself included) use Twitter. Other people (such as myself) use it differently. I use it personally to keep up to date with tech developments (using a Github filter) and to get rid of things I don't want to see (e.g. filtering out #xfactor). Every person is different though, and unfortunately it seems like Tweetdig would be overkill for what you'd use it for.
Initially the idea of filtered tweets seemed compelling. I think everyone has that one spammy friend who they only follow because unfollowing would be impolite. But soon after signing up I realized that it's simply too much work to be worth it: not your app specifically, but using a separate client and having to come up with filters. Filtering and organizing is simply not natural for Twitter - it's meant to be a continuous stream where missing something is no big deal. I could possibly use filtering if it was in a form of a browser plugin (so I could still use twitter.com) and had just one option - to suppress tweets from a specific account.