My wife and I regularly check-in to retail stores, restaurants and night spots via Foursquare and Facebook. This isn't normally newsworthy for me, but yesterday it proved to be influential on a large purchase we are planning.
Shortly after leaving the retail store of a specialty mattress manufacturer, I noticed that a few comments were left on my Facebook status (which I previously used to check-in) cautioning me to not proceed with the purchase. The comments ranged from tales of manufacturer defects, loopholes to their warranty, and suggestions for alternative mattress manufacturers.
It's funny, because I typically study and share research with my clients about consumer social networks influencing purchase decisions. Guess it's... validating, to experience something like this first-hand.