1) Change your name. Tweet + Digg sounds like a social media parody. Also, I don't think you can use the word "tweet" in your name without risking being sued by Twitter. And, if you grow, you may expand beyond filtering Tweets.
2) Your audience and customers are people that use Twitter for business - small businesses, marketing departments, ad agencies, etc. Take a look at other firms in that space (SEOMoz, Buddy Media, HootSuite and see how they do things. Right now your language is geared towards consumers that like Twitter. Change that. Your website needs to explain to a small business how you can help them make more money.
3) It IS a good idea. Social media filtering is a big problem. You are just selling it to the wrong people.