"carefully selected third-party companies"
Translation: anyone who pays them for the info....
Just like the (e- or snail) mailing lists sold on by pretty much every company in the country if you do tick the box / don't tick the box / tick the box with too small a tick / tick the box with too big a tick / tick the box with the wrong color pen.
I don't (for once) understand the problem. Every time you hand over your info to any commercial enterprise, you have to specify that you don't want them to pimp your data out. Is what FaceBook is doing too different? As long as you have the opt-out option, how is it different from what everyone else from banks to supermarkets do, aside from the volume & nature of the data they want to flog?
If you use a Nectar card, Sainsburys want to sell details of your shopping to their Nectar partners - I find that more invasive than if I had a FB profile and Zucker wanted to sell details about how many friends I have on my list or what sort of stuffed toy I am...
(Note: I don't have an FB account, and I'm no fan of / apologist for them. If there's a blindingly obvious reason which is only apparent to FB users, forgive me for not knowing about it!)