I believe that, given the success of a ridiculously large number of fast food chains (and the lack of a monopoly on any of the various "genres" of fast food), there is a massive economic bubble pertaining to fast food restaurants. They make billions of dollars on a daily basis, and people never stop going back.
I have read that the popularity of fast food restaurants is declining steadily, as people are more wanting to go to restaurants that are more home-style cooking, somewhat in between fast food and fancy sit-down restaurants. They want the healthier food options, as well as the home feel to the food and setting. It's possible fast food restaurants will eventually be in trouble as this newer style of restaurant gains in popularity.
Fast Food restaurants have been around for decades. Many of them have thrived and many of them have not. The new Fast food restaurants have tried to take a healthier approach but only some of them will survive. Big Chains with good products will continue to make money well into the future.
No, I do not believe there is an economic bubble pertaining to fast food restaurants. There is so much competition among fast food restaurants, not to mention competition from upper scale restaurants and simply from people cooking their own food. The money that can be made by a fast food restaurant therefore seems to be limited, and Wall St. should not get so excited.