Allow me to add a little bit more. In the beginning of any product cycle, it's not going to have a massive user base right away. It takes time to build up the fan base. It's guaranteed that the user base of the Spa track is only going to increase, so don't fret, only more and more rooms will appear online, not less. Unless these users decide to stop playing Spa online or GT5 altogether, but that's a different issue.
With free access for the first 2 months, besides the reasons "I want to support PD's decision for future DLC" or "I want Spa for offline practice/arcade mode" or "I want to be a Spa host and play with track settings", I don't really see why anyone would pay $7 or whatever amount to buy the track when you can just go online and find plenty of rooms to race in for free.
That's why the current user base is so small, because people could race without paying in the beginning. Why pay up when you can get the same utility (race on Spa) for free? Just wait for a while, and if your GT friends still don't get the Spa DLC, I guess Spa just isn't important enough to them to spend money on.