They're not selling personally identifiable information. They're selling generalized group info on categories people fall into for targeted ads.
I don't understand how this is a privacy violation or unethical. Especially given Facebook isn't hiding what it's doing, they're open about what they're doing. I assume the customers are informed and are consenting to this in exchange for free use of Facebook's services.
I think it was wrong for Ubuntu to sell user data to Amazon because their users weren't informed. Some people choose Linux for privacy reasons and that trust was violated.
No one who values privacy would willingly use Facebook. Facebook doesn't hide what it does, it's a consensual sharing of information. They even have a big button on their site inviting their users to try targeted ads. Everything they do is in the open.