At the moment the twitter account I use for my forum has 851 tweets. Most of these are posted via an RSS feed from my website every 12 hours twice a day with the 5 most recent active threads on my forum
I have a site created in 2004, before Twitter existed, that has over 180,000 re-tweets because I promoted it heavily when Twitter was launched but it's done me absolutely no good in the long term as the extra traffic from these is very brief. A re-tweet from 2011 for example will not get you even a single visitor in 2013.
Currently at my website BankwithPL, it's got over 200 tweets. Though I don't use it regularly, people are still commenting, re-posting, or promote my site. It's very nifty because then it will allow me more traffic.