Mashable has some nice stats today on the popularity of various viral sharing services and email. Here in Professional Services land at Brightcove we've integrated with Clearspring's AddThis widget quite a bit, which is a nice one-click way to post a link to just about every viral/social/bookmarky service out there. I generalize and say "just about every" because under the "More" button they've got services like Segnalo, Propellor and Plaxo that don't exactly set the world on fire.
AddThis is nice, but invariably, clients seek to get a few chiclets right there adjacent to their video players - a clear call to action for a few key services. But the question often comes up - "which services do you recommend."
For the past few months we have been recommending Facebook, Twitter and whatever else is important to the client. The stats seem to back up those recommendations. If Yahoo were somehow able to combine its various properties - Delicous and Buzz for example - that would be another interesting contender.
Twitter links seem to have a pretty high clickthrough rate, which can be interesting and worth considering as well. Mashable has some additional stats here.