I've been on the hiring side of the process at multiple companies, big and small, ranging from being interviewer to hiring manager.
At smaller companies on tighter teams you may occasionally google candidates but you typically don't have time to dig into the details of anything they've written.
For all of the larger "day of white boarding interviews" type places they absolutely don't bother to read up on you. The people interviewing you are annoyed enough that they have to take a break from work to interview, and depending on the location the hiring manager might not even get to meet you before the offer is made.
And if someone does read deep enough in your work to come across this then they're just as likely to have this tip the scales in your favor as not. I've certainty had candidates that I've wanted to hire more after seeing controversial posts, and have been hired by many people who feel the same.
You most likely gave them your e-mail. Also often (in this industry, at least) online username can be gleaned from Facebook or LinkedIn URLs.
Having that, it's trivial to discover your wider on-line presence if you have one (and never bothered hiding your identity).
As for who would bother doing that? I guess it depends on company hiring practices (particularly on the amount of interviews), but in every company I've worked in so far, someone would bother doing a little research on a prospective interviewee.
And you’ll never know that they did.