In the past 2 months I have started 4 different Twitter accounts linked to 4 different sites.
One is brand new, so it has hardly any followers.
2 are less than a month old with over 1,000 followers, and they do send some traffic to their respective sites and a few sales have been made. These sites are basically on idle until I can get around to actually SEOing the sites and promoting them more. The plan is, when I get started on them (marketing) that I will have a Twitter base to play with and get me going.
Another has 2,000 followers, but I have been careful to target as much as possible. It does send traffic to it's intended domain, some days Twitter is my main referral depending on the post that day.
I tend to stay away from "get a lot of followers" schemes. A lot of followers won't do me any good. I need people interested in what I am selling, besides, I am so sick of auto responses on Twitter that say "
Thanks for the follow. Learn how I got 10,000 new followers in just a few days, click here". I don't do that to people anymore, and have retained just about everyone....it is very rare that I will drop a follower since I stopped that madness. It seems like it's 80% of the new people on Twitter. I probably could have more followers, but I make it a point to block those people out and not follow them..all they are going to do is spam me with offers.
Most of what I do on Twitter is automated (who has the time), but I have my accounts up and running on Tweet Deck, or Tweet Grid most days (that's where multiple monitors comes in handy

), to respond to people and not look like an autobot. It actually works out pretty well....my sports account gets a lot of retweets from MLB and Different team accounts (the real ones).
Only on one account do I actually tweet my own stuff, but that was it's purpose from the beginning and was made perfectly clear. I also do giveaways and run contests on that account.
Has Twitter made me money ? In a way. I can trace direct sales, and traffic from Twitter, but more than anything it helps give me branding to people I would have otherwise not found..and it's kind of a nice way to pass the time, even when you are working.
On average I'll pick up 7-30 new followers (targeted) a day, per account, but I do drop a lot of people who are just looking for followers.
For instance, I have one account that is all Detroit Sports...I only want to talk to people that are from Detroit, Fans of Detroit Teams, Fans of teams that hate Detroit Teams, Fans who used to like Detroit Teams, People who went to college in Michigan, Sports fans in general, and so on. I don't have time to talk to people about marketing, or blogging on that one...It's for sports.
When you try and target your followers, it is much easier to keep up your responses across many accounts because you aren't wasting time talking politics on your NASCAR account, or talking about Sarah Palin on your Dance Music account...but that also will be determined by what you tweet, and keeping your accounts separate.
In my short time on Twitter I have learned that you get more traffic by not trying to get traffic.