There's also the data aggregation business that comes along with it. That in itself has become a huge revenue stream for the biggest of the sites on the internet.
Some sites will share the information you provide with these "partners", i.e.- trade. They agree not to sell your data to third parties, but that isn't legally considered their "partners". After the partner gets your data, who knows what happens to it. That partner has no agreement with you as to who they will or won't give the data.
I worked in data aggregation, and I've seen first-hand how it works. Even the dark side of it that includes how business deals get done and data gets "acquired".
That data can make a LOT of revenue. When you can provide someone's name, email, location (not even physical address is necessary), companies can use that to "beef up" (essentially improve the quality) of their data and sell it as "fresh" to people looking for the latest data.
It can potentially make a company $10ks to $1Ms per month/quarter of revenue depending on their customer/user base.
Like I said, I don't remember what the policy here states and how the legalese is written. I read so many all the time when having to sign up for sites. I keep over a dozen spam email accounts just to use for that at home, and I use my work email for a lot of professional sites for work so that I don't have to get the spam at home. Let the work email admin worry about blocking spam.
I do have to say though. I receive no spam at my work email after signing up for SQLServerCentral (and another site I found here where I attended some online education/seminars). So, kudos to you guys for that.
If you saw a couple of my home emails and the volume of spam they get, you'd understand why I so appreciate my work email not getting pounded with spam.
I do understand tho the balance that has to be maintained in making sure you remain viable both professionally and fiscally. It's not easy by any means.