The Backpage case is wild (and has been going on for five years). The site took ads for escorts (which is legal in much of the country, although there may have been ads for prostitution too incidentally, which is far less legal, although still consensual and it's not clear they knew of these).

However, because there may have been a few cases of sex trafficking, they want to hold the site owners liable. This is despite the site co-operating with the police to bring these traffickers to justice.

This is in spite of the Deputy of the Department of Justice admitting back in 2018 that prosecuting Backpage would just drive people underground, and make sex workers much less safe. Indeed, a recent report by the GAO has admitted to that much.

They could have just never taken up this case, or dropped charges after it became ever more evident that this was a terrible idea to begin with, but they insist on pushing this through.

As you would expect, the people who ran the site plan to invoke the First Amendment as a defence. They haven't gotten far enough to use it yet, because of all kinds of delays and prosecutorial misconduct (leading to mistrials).

So, it will be very interesting to see how that goes.