Location based social network Foursquare announced this afternoon that it’s raised $20 million in very high-profile venture capital. We wrote yesterday about why people like to use mobile apps to declare their off-line location: they say it’s for serendipitous connections, tracking friends and family, for the gameplay and for recording their own personal histories like a lazy diary. It’s probably also a way for many people to brag about the places they eat and shop.

The company’s backers say that founder Dennis Crowley is the thought leader of the location based social networking world. Investor Ben Horowitz blogged today that Crowley “not only created the vision for the company, but for the entire product category.” Crowley addressed the geogeeks’ conference Where 2.0 this Spring and the 20 minute video of his talk there is probably the best place to get a feel for the man and the vision behind the company.

So what do you think? Might this man’s company change mainstream commercial culture more than Yelp has? Yelp is 15X the size of Foursquare so far. Can it withstand the coming of the Facebook Location Feature? Facebook is today 250X the size of Foursquare. Crowley offers examples of businesses that love Foursquare, but there are plenty of business owners who are annoyed by or hostile to it as well.

I like Foursquare, myself. I enjoy using it and think that there is incredible potential for location to be a platform for the development of incredible new services. We’ll see how it all plays out.