You can't keep up with everything that is going on in social media, but you can try. I have my regular sources and favorite books, some of which I mention below. My list is by no means exhaustive, and is meant to be interactive -- so please add your suggestions in the comments.
The blogs: http://www.thelongtail.com, www.mashable.com, www.techcrunch.com, http://publishing2.com/, www.paidcontent.org, http://news.cnet.com/the-social/, www.avc.com (Fred Wilson), http://adweek.blogs.com/, http://web-strategist.com/blog/
The staples: Wall Street Journal, New York Times, Businessweek, Wired.
The trade pubs: MediaPost (of course), Ad Age, iMedia, Adweek.
Because social media marketing can learn so much from search: www.Battellemedia.com
For what the kids are up to: www.Ypulse.com
Best clipping service: www.iwantmedia.com
And a couple of books for good measure: "Greater Good: How Good Marketing Makes For Better Democracy," for what marketing can and should be. "Competing on the Edge: Strategy as Structured Chaos" -- more general strategy for business, but the best strategy book since Porter, and has tons of implications for today's marketer. "Sway: The Irresistible Pull of Irrational Behavior" -- nothing like taking a little time to understand people's motivations.
And, of course, my Twitter feed. A lot of smart people "micro-blogging" their every thought.
This is just a sampling, and like any good consumer of media 2.0, I will follow the links to the stories that keep me informed. What are you favorites?