Social media is easy to measure. That’s one of the most compelling factorsof online behavior – it’s trackable and comparable. And for marketers that means we can combine the creative right-brained activities with the logical left. We can fly by instruments, not just gut reaction. And we can demonstrate progress. That’s not the same as Return on Investment, but for many companies, it's more feedback than they have on other areas of the business. Measurement helps us learn what’s working, do more of it and do it better.

• Blog – traffic over time, comment numbers over time, posts, comments per post, inbound links, RSS subscribers
  The goal here is to track your reach and basic level of engagement. You can get the traffic and comment numbers from your blog platform. Feedburner will enable you to get the number of RSS subscribers. Technorati's Authority index tracks the number of inbound links (although they've removed Watchlists as a service it seems).