# of visitors
time on site
entrance and exit pages
Take a look at this post - /
http://blog.ogilvypr.com/2009/06/introducing-conversation-impact-social-media-measurement-for-marketers/ - they include some very interesting ideas for measurement, including Sentiment index (% positive - % negative) in social media, Share of positive voice in social media, within category, Relative Net Promoter Score, absolute or within category.
You can also measure action - set up a registration page and then count registrations. Or ask people to click a link to read, then count the number of clicks (this is also called conversion).