The mobile revolution is spawning a variety of business relationships to attract new customers. The latest is the partnership between credit card holder, American Express (AXP) and the social network Foursquare Labs Inc.

Both companies are capitalizing the growing segment of online purchasing. American Express had roughly $100 billion in online transactions last year. Online spending is growing at a 20% rate, compared to15% for overall spending, as reported in the Wall Street Journal.

One of the hottest categories in tech is location-based services (LBS). Essentially, this means using your smartphone's GPS capability to show friends your current location. One of the early leaders in the space is Foursquare, which recently snagged a large amount of venture capital. But other players are diving in. For example, Shopkick recently rolled out a pilot of its LBS platform to allow shoppers at Best Buy (BBY) to get discounts. And of course, Facebook recently launched its Places capability.

In fact, now AT&T (T) is getting into the game. The telecom giant has teamed up with an LBS startup, SCVNGR, which has a rewards system for the iPhone and Android. It will be in 50 AT&T stores throughout the Midwest as a trial program.

Yahoo! (YHOO) and Facebook both tried to buy Foursquare, but the company instead wanted to stay independent (taking a recent $20 million investment from Andreessen Horowitz). Then again, this scrappy startup is one of the hottest in tech. Basically, Foursquare allows people to use their smartphones to indicate when and where they are checking-in to a place, such as a bar, restaurant or nightclub.

When Facebook announced its new location-based capabilities after Twitter has already enabled it, the future looked pretty grim for Foursquare. Though wildly popular with the nerd crowd (of which I'm a member ... the nerd world, not Foursquare), could a year-old location-based game go head-to-head with the 400 million-user-strong behemoth of the social media industry? In a strange twist, Facebook is actually breathing life into the killer app many expected it to kill.

Thirty-three percent of Foursquare's traffic comes from Facebook, according to data from Hitwise (EXPN), followed by Google (GOOG) at 22% and Twitter at 8%. The remaining one third of traffic, from everyone else, is fed in part by partnership with major brands such as the New York Times (NYT), Bravo and Zagat.