I agree that social media will affect SEO, but I have a another different angle on that.
As social media grows, Google understands that the future of search is personalization. As an evidence to that, Google, in the last few weeks, announced that personalized search is now for everyone, and that they are rolling out their social search as an open beta. Both of these announcements, in conjunction with the Twitter feed in the SERP, shifts the SEO focus from a technical profession (optimize page elements, link building, keyword selection) to something that is more social.
The future of SEO lies in networking, as Tara mentioned, but not just because of the content, but becasue Google will try to measure it; Google's algorithm will incorporate your social network into your search results, and it will even give it priority over old variables, your network will become crucial in your SEO efforts, moving from just SEO to a Word of Mouth Marketing, and making number of followers, fan base, and subscribers to heavy weight elements in a website race for rank.
In addition, first impression will be much more important than what it is today, as bounce rate and personalized search will determine if you will appear on a person's SERP in the future. This creates a problem for SEO because first impression is mostly visual, and SEO can't really measure that.