This site, Dopplr, was created for use in Europe and the site it only mentions activity in New York in North America, but one of our members is using it for travel in Florida.
It's a social media application where friends can share travel tips and information about where they have been. It's free and I love to have your feedback on it.
Edit: More Info -- Here is jpeg of an annual report posted by Sean Bonner that clear shows that one can use it in the USA -- I like the carbon footprint element as well. (I can't link to the actual report -- because Sean hasn't provided the link and it's private).