The Foursquare and Yelp campaigns are similar in that they’re both humorous. The Foursquare ads use words that are different but sound similar to illustrate the different wants of the people in the ads which makes it interesting while the Yelp commercials use a particular kind of humor (showing people who are in uncomfortable situations and need a solution, that the man offers, which seems like the obvious option) to get the viewer to think similarly. Yelp is known for its reviews and there is a strong sense of trust involved while Foursquare has a much stronger social sense since you can look up places to go and then check in at these places. The audiences are different in that those looking to use Foursquare are concerned with a more social aspect than those looking to use Yelp which is more of a source for trustworthy advice.
The tone of each of the campaigns speak directly to their audiences just as the photography style does. They chose to shoot scenes in real environments but for Yelp, they make sure the actors interact with the viewer so that you can definitely relate to them. For Foursquare, the models were shot going about their lives and not looking at the camera which shows who the viewer could be if they used the app.
I think your suggestion that trust is really important in the yelp ads and less so in the foursquare ads is an interesting one. Yelp is positioning itself to be dependable. Foursquare is positioning itself as discerning. Good distinction.