When Foursquare first came out, it seemed like just another silly social app for iPhone geeks, but now that it has had some time to grow, it’s becoming a must have for just about any business.

Major players like Zagat have signed on and will be doing more than just providing badges (the way users boast to each other on Foursquare). The foodie staple will be interviewing the mayors (biggest visitors to a venue) of Zagat-rated restaurants and will also suggest local Zagat restaurants to users.

Foursquare has also penned deals with Bravo that correspond with network shows; with the new star vehicle “Valentine’s Day;” with HBO’s “How to Make it in America” — even Harvard University.

If you aren’t already on Foursquare, you should be. No matter what kind of company you have, the old adage of marketing is “meet your customers where they live.” And your customers are on Foursquare.

A lot of companies will get on Facebook and Twitter and say they have a diverse social media portfolio, but these secondary social sites are key to getting to as many people as possible. So instead of playing on Facebook for far too long and calling it work tomorrow, get on Foursquare, Mobile Spinach, Whrrl, Loopt, GraffitiGeo, Hotlist, Yelp, Qype, Brightkite, not to mention Google Maps.