While there is nothing inherently wrong with making a profit from data about users, there is quite a bit wrong with the way it's being done on Facebook, and much of the rest of the web/mobile for that matter.

For one thing, services fail to properly inform their users of what they're doing. It's quite common for ordinary users to either not know that their data is being collected and used in those ways, or the extent of the data which is being collected (e.g. Location, Phone Identifiers, and sites visited across the web).

For another, opting out is generally designed to be quite difficult. Unless you're vigilant enough to know which hosts to block (among other things) it's almost impossible to keep your browsing history out of the hands of the hands of third parties (especially with all the different ways browsers can store information, see, http://samy.pl/evercookie/). This is the case even if you do not sign up for or use a service like Facebook.

Also, as someone noted below, there is an issue with the simple fact of the data's existence, since from that point on it is out of my control, and can be used by anyone who can get their hands on it, in any way, which may well be harmful to me.