Consideration was always in interesting discussion by our company's general counsel when going through training.
On thing that stuck out to me was the company would always review entry criteria if it may any requirement beyond what would be commonly available to the lowest ranking member of society. This was typically considered to be a computer with internet access as it was likely to be available at a public library.
While mail has become a more accepted means of non-consideration entry, I believe there was a time where it was considered consideration. Counsel argued that mail-in entries required the submitter to own or purchase a stamp, a postcard, a writing device, and potentially an envelope for the postcard.
An "app only" contest was given extra scrutiny because it required the user to own some sort of smartphone or mobile digital device.
While, that was contests and promotions (in which case, they needed to avoid running an illegal lottery), I imagine a similar argument could be made for WhatsApp. WhatsApp is only available if you own a smartphone capable of downloading the app.
Of course, German contract law is very strange and alien to every other legal tradition (except Japan, I think, and that‘s only because they modeled their civil law on ours).
> A valid contract needs the following elements: People entering the contract must intend the contract to be binding. An offer is made by one person and is freely accepted by another. Some price (money, right or benefit) is paid in return for a promise. 
So the fact WhatsApp is providing a service, means some price has been "exchanged" (for lack of a more precise term). The price does not need to be financial, any benefit can be viewed that way.
So yes, it does appear that a legal contract was formed. Even in the US. 
(A financial benefit is not required, but it makes it more clear that the contract was valid. Hence the habit of ridiculous $1 contracts.)
 See under Consideration: https://www.entrepreneur.com/article/175238
Then Facebook gobbled them up, they removed the small subscription fee (not even giving an option to keep on paying it) and went: "All your data are blong to us".
Ironically thats what I loved about Whatsapp:
I paid them, they provided an awesome messaging client.
No spying. No ads.
I wish they would still offer this as an option, tho I'd probably be skeptical of Facebook actually holding up their end of the deal and not tracking people who pay for WhatsApp.
That was a few years before they were bought by FB.