The fact that Facebook owns WhatsApp makes this announcement a big surprise as I think they profit far more with unencrypted messages (although WhatsApp was just delivering, not storing them supposedly).
The ads I see on Facebook currently are for SSL certificates (guess what I bought recently), BGP routing optimisation products (not sure why), and TransferWise (I used them once). In short, lots of remarketing and something presumably targeted at one of the news sources I get in my feed e.g. slashdot.
These are all pretty nerdy yet also pretty reasonable ads. Ads targeted based on my chats would mostly revolve around .... well, not sure. At best, nights out or local cinemas. I doubt there's much profit in that.
It's also public knowledge that they've been parsing messages for links to then 'like' the appropriate pages inside Facebook.
Rather than debate about how much profit there could be in it, which is entirely down to what the user talks about, I think it is important to realise the potential here even if it not fully realised at this point it time.
Also remember that they are keeping chat history indefinitely in HBase clusters (because they are 'sentimental') so as your habits and interests change over time this will become invaluable data to marketers who want to understand consumer behaviour.
I think it's great that WhatsApp are implementing end-to-end encryption but their now immediate connection with Facebook doesn't sit well with me. Perhaps I'm an idiot, but it doesn't take long to realise that there are perhaps competing interests at play here.