Excellent article forwarded to me by a social media buddy a few minutes ago. The english/grammar isn't perfect, but the points are very sound as the author compares/contrasts the traffic you can receive from social and the traffic that comes from search engines.
His ultimate conclusions are very similar to what we've found: social media (i.e. digg, reddit, stumbleupon, etc.) certainly has its positives, and you want to have a presence, but it requires a lot of work to really drive consistent, significant traffic. Search engines, on the other hand, tend to drive visitors that interact more with your site and that spend more time with your content. It obviously requires a lot of effort to ensure that your site has good SEO fundamentals, but often this effort has a greater ROI than the time investment for social media.
You can't forget about either, but the article suggests the path we typically follow: more time, focus on SEO.