What should I think about these unique certificate serial numbers
I would think these numbers would have been put on the google search engine, and yield (probably) many hits.


This assumption is wrong. Certificate serial numbers are not indexed by common search engines, nor are they typically posted to any HTML site. Frankly, I'm not sure why you would assume they'd be indexed.

The Wordpress certificate is used for all Wordpress domains; it is for *.wordpress.com (matching all subdomains of wordpress.com), and has wordpress.com as an alternate name (so it also works there). The Google certificate you list is only used for www.google.com; it is again something that commonly shows up (Chrome reports a different certificate for some reason).

The reason you don't find any posts about them on Google is that no one really cares.