Mary's got a detailed analysis of how services track links to blog up, with more to follow.
She writes: "This exercise is an attempt to give readers and users of the services a comparison of how the services so that they can take best advantage of the strengths and avoid the weaknesses in order to track URLs, keywords, other special services, and alerts or subscriptions or watchlists (the services each use different terminology in order to differentiate themselves but users tell me the terminology is just terribly confusing and they wish that as an industry we would settle on one term and use it across all the services and then get on to figuring out how to provide the service better."

Well worth a read.

Oh, and here's a PDF of the data.