Is it common to cite the numbers of results a search engine yields as evidence for the prevalence of something online?
Do not do this. The "approximate" number of hits Google reports is completely worthless. To see why, look at this number on both the first and the tenth page of Google hits:

First results page

Tenth results page

When I just did this, I got "approximately 9,010,000 hits" reported on the first results page... but only "approximately 48 hits" on the tenth page.

Your results will probably vary, depending on your search engine bubble (another reason why this number is useless).