I know that most technical people can do this themselves, but in case you need something a bit more worked out and documented, here is a link to a great facebook-like button (+free analytics if you buy it at the same time) for facebook. 39 dollars if you use the coupon code he mentions in the video.
There’s another plugin out there that costs only 5 dollars, and it includes Google+ and Twitter, but I felt uneasy on buying it because it seemed unfinished + seems to be based on copying shareit code from the shareit website (where you have to agree to basically letting them piggyback your traffic for their purposes). No thank you.
Installation of the basic functionality is a piece of cake or as they state in my part of the country : 2 fingers up the nose easy. Upload the code you purchased and downloaded from the retailevolved website and upload it to your magento directory. Plus the documentation is included and is very clear on what you can do and any variations.
Plus you can go a step further, and get more functionality like the “send” button if you setup a facebook dev account and get an application id for your website.