For the yelp ad based off of the look and general feel of the ad the demographic is more for a everyday person that might need a quick fix or are willing to try out something new on the go so that way there will be a review for the places they are looking for so that they know what they will be getting into. Clearly a younger more general crowd that can relate to the crazy situations we find ourselves in.
The foursquare feel and take on it is for more sophisticated people that have specific things they like and personality so even though they are at the same place they are there for different reasons but because of what they like brings them together. This ads focus is for people that like specific things not to much into switching things up so they want a good place that meets their needs.
You make an interesting point about the difference in audience between these two ads. You suggest The Yelp ad has a younger appeal not just because of the zany humor but because the people in the ads need help- rhymes with yelp. Though don’t older people need help? I am not completely convinced by this argument. The people in the foursquare ad are presented as already knowing what they want and foursquare will make it easier to get it.