They've been doing it for a very long time. Facebook is nothing but a massive datamining operation. Google listens through your computer/device as well. That's why you see ads tailored to things that you've only mentioned in voice conversations with people. This is known. Amazon does it as well. All the internet giants do it, through various means. There are several topics written about this on my website:

"Advertising" used as a patsy in internet giants' SPYING AND VOYEURISM
http://www.sectual.com/thread-206.html

Amazon gives Google a run for its money, datamining all aspects of its userbase
http://www.sectual.com/thread-692.html

Google INDISPUTABLY just read my mind and I'm totally freaked and not cool with it...
http://www.sectual.com/thread-2516.html

Google is IMITATING natural synchronicity to target ads...
http://www.sectual.com/thread-2520.html