It’s well established by now that Twitter is a ranking factor for search engines. It’s also fairly certain that they use a PageRank type formula to determine the “value” of your tweets. Lots of people following you? PageRank up. Most of those followers are spammers/bots/etc? PageRank down. It’s suspected there is some sort of trust rank as well where if someone “famous” follows you, then that probably increases your trust.
One factor that is a little less obvious but still important: the ratio of followers to those you follow. If you have 2,000 people following you, but you follow 10,000 people, that’s a minus. If you have 2,000 following you, but follow 100, that’s a plus.
So here’s a really quick and easy way to improve the value of your twitter account. Simply unfollow all the dead accounts you are unfollowing. I just did it and removed 43 accounts (out of 147). I had about 50 inactive accounts I was following, I didn’t unfollow a few people who I would like to hear from in case they become active. I used http://manageflitter.com and it took me about 2 minutes. Try it, and hopefully increase the value of your tweets.