The "Biggest Loser" certainly proves people are passionate about Fitness. Fitness is something that is ongoing (ie regular updates)-the type of info people add to their Facebook feeds already.
There are lots of fitness apps--but no fitness social networking specifically for the clubs to offer to clients as a form of lifestyle marketing.
This borrows on the Cloud computing/SaaS model and would be sold on a monthly/yearly basis to fitness clubs. It also could be integrated with a platform for virtual conferencing via synchrous or asynchornous training. The later might appeal more to moms who are stuck at home.
I have a little problem wrapping my mind around why a user would want to use a local social network provided by a Fitness club vs. say Nike. However, you are building around existing relationships and provides a potential backbone for finding workout partners (who actually uses your gym or workout facility)
Fitness as a business is certainly increasing.
A paradoxical case from Google trends on
fitness makes the case that fitness clubs have to get users either in the January to March surge or the rest of the year. Providing a unique value add for fitness clients could perhaps help Fitness clubs keep and gain.