Just read an interesting case study on traffic conversion with a dozen traffic types – AdWords, Twitter, etc.
While the sample is relatively small, the results are interesting, particularly how Twitter Auto Follow (by setting to follow the competition's followers and sending a direct message to everyone who follows him) beat all the competing traffic sources for visits and conversions – although Social Media and forums beat it on conversion rates.
One other advantage for the Twitter Auto Follow method is that it is self-sustaining. While forum posts sometimes get you traffic years after the post, most don't, so it requires constant attention on your part…
