The ShareThis team has posted a set of Share Icon Guidelines as an RFC.
I’ve watched this go through a few different revisions and provided my input along the way. I think the guidelines are excellent1, they keep the icon free for everyone while helping define how it should be used, so that its meaning is well preserved.
If you’d like to comment or send feedback to the ShareThis team on the guidelines the best way to do so is in the Google Group. Kudos to the team for doing this the right way.