I've been contributing since 2004, though I took a break the years I didn't have a Thorn bike, I don't recall it ever being very busy. Some people join, make a lot of posts in a fairly short time, then disappear. That's the way forums are, a few get enough momentum to form a critical mass, but most stumble along or fizzle out. I think that this one survives at all is largely down to Dan's stewardship.
Facebook is much more lively, but IMO feels less personal, plenty of Thorn discussion across several groups and despite this forum carrying the name, you're more likely to hear from Mr Thorn or Thorn staff there rather than here.