I would argue that most people who had a blog were 15-25 in that time. Yes it was very common in that demo, but outside of it, it was definitely not. I don't know if that classifies as "mainstream".
The very active ecosystem of blogs I followed in the first decade of the new millennium, on arts themes (literature, cinema, non-popular music) and religious-denomination news, were mainly people above 30 blogging, sometimes much older. Wordpress had made it easy for any computer user, not just tech nerds, to set something up.
