I got it from "add this", you can get code for adding to the bottom of each of your posts it's very straight forward just follow the instructions. There is a step by step tutorial and then once you hit save it's there at the bottom of all your posts. They also have analytics so that you can keep tabs on it, I haven't really investigated that side of it yet but could be interesting - there are thousands of social networking sites, it's quite overwhelming!
I also added it to my sidebar (again it's very easy use the instructions on the 'add this' site) but I'm really not keen on all these boring looking buttons that are starting to appear in my sidebar so I did a bit of fiddling with the html from the code they gave and changed the one on my sidebar to an animated stirring coffee cup which I thought might also encourage more people to click, coffee encourages me to do a lot of things! It does exactly the same it just looks prettier.