I started using Version 2 just after it was released. In those early days SketchUp wasn't very well known. It was just starting to get noticed in architectural journals. It was a brief mention in an article in Canadian Architect that had me curious. Although really not all that long ago, in those days relatively few people had any kind of high-speed Internet so on slow dialup's it took some motivation to do the big download. I remember telling many people about it but they couldn't be bothered to sit through an long download. Also I think many then still didn't really believe 3D modelling would be all that useful or that it could ever be easy enough for them to learn casually. At that time all other three-d programs had huge learning curves and you had to be very dedicated to struggle through lots of trial & error just to get anywhere. In the work environment 3d modelling was far too time-consuming for most firms to even consider. It also was associated with presentation work and not design.
SketchUp changed that. Even in version two it was quite stable and very practical to use. All the basic functions were there (I think). Those who would try it would find that they could learn it easily, it was fun, it allowed creativity to blossom, and it could fit into one's work-flow. It was word of mouth and tradeshow demos that saw the numbers of users grow. There was almost no advertising. By the time Google bought it I'd say it was well known in the Architectural community. For us I don't really think it is any better known after Google bought it. The release of the free version has allowed it to reach new users in the hobby arena. Those new users would never have bought a $500 version.
Regards, Ross