I found Twitter to be pretty useless. At the moment.
I am getting customers to tweet that they are using the site. Some do, some don't and it has helped, so far in a small way to reach followers of my followers but I don't invest to much time in it at the moment. I share blog posts or interviews I have on the site and some people come and look, but not very many according to stats.
I feel, to a degree, people/small businesses are just too busy promoting themselves on there.
It's a slow burner. I wouldn't worry to much about getting business from it, it will just frustrate you. Tweet relevant things related to your business. Whether that is directly linked to your site or articles you've found interesting elsewhere. I think that's the way to use it. Engage with people, who want to share the things you say and their followers might find you then.
I haven't found twitter of FB very useful at the moment for the amount of time you have to put in to it. Build it up slowly and eventually you might see a return / your visitors growing..
For me. As above, I just post things every now and then, instead of tearing my hair out focusing on getting more traffic and people from there. My market is brides and the only way I could get brides from twitter is to actually add them as a friend and msg directly, which I am not going to do and is a huge amount of time which would just end up annoying people.
Personally. i think you've just got to build slowly and the goal is to get people to find your site naturally. You can take a horse to water and that's about it