It still requires you to run the WhatsApp phone app though. I doubt that facebook wants to require all of their users to have phones. You can reimplement WhatsApp's functionality in client side JS but then you get into deep technical problems rooted from the fact that js is, in general, ephemeral and not something permanent compared to an ios/android app. E.g. when someone logs onto facebook on a their friend's computer to check their messages you want everything to be smooth including message search but in order to provide message search, either the server needs access to the plaintext or the client needs to download the entire message history... If there's an app the client already has the entire history so that's no problem, but it is a problem with ephemeral Javascript. Another problem you run into is that Facebook can serve specifically manipulated Javascript just for you ^ TM because you are an interesting target or something. For Android/iOS this would require an app update and need to go through a third party and I haven't heard that google or apple give you the option to push a specific app to a specific device.