But try it for yourself, open facebook, back out, wait a few minutes, task switch back to it. Itll be updated without having to pull down to refresh, sort of in the way android handles multitasking but a bit cleaner in the sense that if its not in the switcher, its not really open.
I'm not willing to go out on a limb and say you are wrong. You could be right. However, I don't think that your observation necessarily leads to that particular conclusion.
Windows Phone has always supported the delegation of very specific background processes to the OS. Tracking your location, playing musing, transferring files, receiving push notifications and a few other things have always been able to run in the background, even when the app that controls them was fully suspended. WP8.1 made a lot of changes in this area. There are now many new types of app controlled processes which the OS can run in the background, some of which cater to very specific types of applications (like WhatsApp or Facebook). I suspect that what you are observing is one such capability specific background process. These are very resource limited and time controlled processes that are managed by the OS, which can run even when the owning app is fully suspended.
Like I said, I haven't read through all the docs yet, but WP's restrictions in regard to multitasking were put there for a reason. WP wouldn't be as fluid as it is, particularly not on low-end hardware like the 520, if MS removed those rules and introduced the free-for-all (do and break) whatever you want Android model. It's possible, but very unlikely.