Oh this is funny, remember when there were like all this influx of buttons to save/post/share/like and then came the aggregation buttons, like addthis and addtoany or sharethis, all of this was a freaking drag on webpages and i was never a big fan, cause they weren’t really very useful, sure 1 or 2 people might use it, but the vast majority doesn’t, and why make your pages load like shit just to make life better for a couple of visitors?
Most people that use delicious have and use other ways to bookmarks, users of stumbleupon probably have the bar installed, so there wasn’t any point, that is until facebook started with the like buttons, those seemed to be way more practical and a stronger promotional tool, same goes later on to the tweet buttons and now the google plus button, so we are back to the same problem, although the buttons are more useful and practical its still at least 3 or 4 hits to different places dragging your own pages down (ohh and the tweet button is kinda the worst of the bunch).
That is were addthis comes along and just adds these buttons to their standard setup, so now instead of all those hits you are just hitting the addthis site, add to that asynchronous loading and you are pretty much having the best of both worlds, adding to this the fact that i dont need to hardcore parts of the template or some referral on the buttons to make them work properly, makes addthis even more practical, so expect some more buttons popping out of s2r sites hehehe.