Something I heard about recently is that it's unnervingly common for the stock prices of unknown but really important companies like these to shoot up following an outage because it reveals to stock investors how mich of a monopoly it has in an area.

Are you not in the US? Equifax is a credit bureau and if you've never heard of them, you never needed credit or you're not from the US.

The other three, I've only heard of Ashley Madison because they had a very aggressive ad campaign before ad blockers became ubiquitous. One could say it was ads like theirs that made ad blocking a requirement.