Fitango is a fitness and goal tracking web site that goes beyond merely logging your progress and offers guides, expert-created plans, easy tracking, and peer-based motivation to keep you moving towards your goals—fitness-related or otherwise. (lifehacker review)
You can create an account using Facebook, which allows you to share your results with friends. Then you either create your own workout or you get one through their Planstore. There are also other categories to get challenged in, but here's an example from the Fitness & Sports department:
For this example I picked a marathon plan, you can invite Motivators. These are friends, trainers or fellow athletes, that can offer advice, motivation and help you stay on track!
You have to manually keep track of your workouts, which you can then easily publish to Facebook. You can get an overview of your workouts on your Report tab, which gives you graphs and other info.
Luckily they also have an app, which means you can update your Actions from your mobile phone, rather than requiring a computer.
As for any further social activity, there's the Bubble.
The Bubble is a place for all users of an Actionplan to discuss their progress, challenges, and experiences. Using the Bubble is simple. Just write something in the text box and click Post. Remember -- whatever you post in the Bubble is visible to all users of that Actionplan. You can easily attach photos or include links in your Bubble posts by clicking the appropriate icons.
Fitango has a ton of options and since it's still a beta product, they will probably add a lot of new features in the future. One downside compared to apps like RunKeeper or Endomondo, you have to manually fill in your Actions. But in return, it has a lot more social features and let's you keep track of virtually anything!