I'm stunned by the traffic figures as I don't know one person that uses it (and I suspect the majority of my non-tech, Facebook loving friends have never heard of it).
Is that anything to do with my demographic (41, M, UK based)?
Yes (demographic). Although all sorts of people use tumblr, it seems to be most popular among (predominantly American) high-school age (and a bit older) teenagers. It has settled into a middle ground between Twitter and traditional blogs for a lot of these users -- single-paragraph posts or photo posts are the norm, and re-blogging is very common/encouraged.
Is that anything to do with my demographic (41, M, UK based)?