Although both Apple and Google have friend location apps, it's interesting that both companies have been shy about using them aggressively.
For example, neither Find Friends nor Latitude make it easy for you to track friends. First, you have to enter each friend manually. They receive an invitation, and must explicitly accept it, and also must run the app in question. In the case of Find Friends, that means they must be an iPhone user.
Given Facebook's demonstrated desire to get people to share everything automatically, it could be that Facebook may enable you to simply invite all friends at once and try to get mass acceptance very quickly.
Similar apps like Highlight exist and are considered somewhat invasive. Highlight, for example, notifies you when friends are near, and also keeps a history so you can compare your wanderings with those of friends.
But if Facebook implements some of these features, explicit tracking may go mainstream much more quickly.
And, of course, if Facebook succeeds with a tracking app, Apple and Google are likely to match them feature for feature.
Constant location tracking, of course, happens now. The difference is that the "circle of trust" over who gets access to your location information is widened, and the potential for abuse by companies, governments, hackers and criminals is that much greater.
Still, I think it's an easy prediction to say that most smartphone users will eventually run these apps because of the benefits for mobile social features, convenience, money saving and advertising relevance.
Whether Facebook is the first major company to do it or not is irrelevant. Sooner or later, nearly everyone will be using social tracking apps. Probably sooner.