Foursquare, the rapidly-growing social networking service through which people check in at any number of locations, apparently had a pretty good year in 2010 — and it’s no shock (at least to us) that D.C. was a big part of that success. According to an infographic posted on the service’s blog today, Washington was the site of the service’s single biggest check-in event (last October’s Rally to Restore Sanity and/or Fear, where 30,525 people checked…