Anyone with your number, and it reports when you have the application open. Assuming WhatsApp is the first and last app you open in the day (good morning and good night messages) you know how long a person sleeps, and this is only a very easy analysis you can do. I am sure you can find something more interesting recording when people open an app.
OK, yeah I agree that should be configurable, or at least limited to contacts. Maybe it should only count as online if you're actively using the app instead of having it open in the background. I'm sure many people just have it open 24/7. If I still used it, I would want an option to only receive messages when I'm online.
>Maybe it should only count as online if you're actively using the app instead of having it open in the background
Not sure about desktop clients and not sure about what you mean by "open in the background" in this specific case, but I can answer this for mobile WhatsApp apps specifically. For those, it only tracks when you actually have the app open in your main view.
More specifically, if you just have the app in the background while using some other app, it doesn't count as online. If someone sends you a WhatsApp message and you receive a notification but don't open the app itself, it doesn't count. Only when you actually open the app is when it shows you as being online.
Personally, I agree with you that the simplest solution that would already resolve a ton of those issues is to simply only display your "online" status to those who you have added to your contact list (instead of to literally the entire world).
