Many people leave multiple tweets re the same url. I think too many times in a row might turn some people off, but going in a couple times a day and seeing what works for you is good. Some people send one every fifteen minutes or so, because people usually just read the first page or two and it's easy to miss a tweet.
As far as unfollowing people, the more people that see your Tweet the better you are. When I as new at Twitter I thought about unfollowing a few people, but then I finally realized, at least I think, their people also see my tweet.
The power of Twitter is in the linking and being seen by as many people as possible. It took me a while to figure some things out about Twitter, and I don't think I have even scratched the surface.
I am rather anti-social also, and I like the fact that I am not out in public socializing. It's easier for me on-line, and even that took a while to break down my fear.
Good luck with Twitter. I am interested in hearing your results.