I wouldn't trust CrunchBase for reliable trend data because its coverage over time is unlikely to be consistent. But, a similar analysis based on legally-required (and thus comprehensive) SEC Form Ds might give stronger insight.

At least two Form-D watching services have been mentioned previously on HN:

http://stealthmodewatch.com

http://www.formds.com

Though, I think they're relying on easier electronic access that may not go back more than a few years.