If you haven't taken an interest in web analytics yet, there are two websites that may provide a spark for you: Compete and Alexa.
Compete
Compete provides a free service that allows you to compare the traffic numbers of different websites. Essentially the service tracks the behavior of 2,000,000 Internet unique users, runs their behavior through an algorithm, and offers up it's estimate of a site's traffic. It works pretty similarly to Nielson ratings. (So, uh, if you are one of the people Compete is tracking...please come back. Often.)
A cool feature of Compete is that it allows you to get an idea of a typical user's behavior on various sites. For instance, thanks to Compete, you can gather stats on Facebook such as: Ave. Time on Site(14:08 minutes), Ave Pages viewed per Visit(44..3 pages), unique visitors per month(26.2 million people).
Alexa
Alexa also provides traffic data on websites but it focuses more on a site's traffic rank than user interaction. It is always good to compare Alexa stats against Compete, and vice versa, to give yourself a range for stats because neither site is perfect. For instance, whereas Compete states that users visit 44.3 pages on Facebook, Alexa says that users visit only 31 pages per visit. This 13 page difference is quite significant and therefore you should utilize both sites if ever using the services for a competitive analysis.
The best feature of Alexa, in my opinion, is the "Sites Linking In" section. This will show all registered sites that are linking back to the site. Essentially, the more websites that link to your site, the better your site will do in search engine rankings and quality rankings.
These two websites are great but there are still a lot of services/features that have yet to be developed, such as a company that provides free insights into RSS feed readership for various blogs/websites. That would rock.