I agree with the thread starter. Having owned several BMWs (E46, E93 M3, now F02 7-series) I've noticed a severe drop off in discussions for this, my current vehicle. By FAR the most active section I've contributed to is the E9X M3 forum. Lots and lots of stuff happening (or happened) there -- got several tips from folks, and contributed many of my own.
This 7-series thread is a bit of a different beast. Probably due to facts already mentioned, but bear repeating (however stereotypical): 7-series owners may be in a higher income bracket which, by extension, means their work days may be a bit more taxing with regard to free time. <-- this holds true for me. When I had my M3, I was in a mid-level management position at my company, and had a bit more "free" time to hit the blogs, etc. However, I received a significant promotion (and commensurate increase in responsibility) and have next to ZERO minutes of idle time at work now -- lots and lots of meetings where decisions need to be made. So -- fewer posts
To be honest, I was actually happier back in those M3 post days. The sense of community was tremendous. But, such is life. Onward and upward.