I can certainly see how Facebook could monetize Whatsapp's data, but surely it isn't legal for them to retroactively change the ToS and examine the data, right?
Most ToS state that their terms are subject to change. I do not know whether or not a company has to explicitly state that the terms provided are perpetual, but there are lots of examples of companies changing their ToS retroactively.
Pretty much all ToS have a clause that states that the terms can change any time. Where services differ is whether they'll provide you with advance notice (or if you're expected to check for changes yourself!), and if you are able to close your account and have your data removed (I believe this is required in the EU).
Looking at http://www.whatsapp.com/legal/, it appears that WhatsApp falls into the "we can change our terms without letting you know" category, and I don't see anything about their data retention policy :-(
For example:
http://arstechnica.com/tech-policy/2008/09/google-on-chrome-...
http://boardingarea.com/viewfromthewing/2012/10/10/united-ma...
And here is an interesting paper about retroactive lawmaking by the government:
http://www.law.harvard.edu/students/orgs/jlpp/Vol30_No2_Weie...