I was digging through our blog's lifetime analytics recently and noticed something surprising: Twitter is the third highest source of referral traffic to our blog.
#1 and #2 are Google and our company homepage.
The total number of visitors that come from Twitter is still relatively small; it's completely dwarfed by search engine traffic, for example. The bounce rate (the percentage of people that exit the site after viewing only a single page) is also fairly high, which makes sense; people will follow a link from Twitter to a specific page, read that article, and leave.
That means dollar-for-dollar we would get more return from focusing on SEO and Pay-Per-Click ads than on Twitter. But consider this: we spend almost no time or money generating Twitter traffic. Our posts are automatically tweeted; when our followers on Twitter see the article and enjoy it they re-tweet it, and that generates our traffic.
There is, of course, time required to set up and maintain a profile, monitor replies, and make new connections, but that is a small time investment. The benefit is a passively generated traffic source that produces steady results.