Now that we've decided Facebook is here to stay, it's soon to be overthrown.
Initially, people expected Facebook to be replaced by a better looking or more exclusive version of Facebook (the way Facebook was a better looking and more exclusive version of MySpace). That never came to pass.
The next big threat was the shift to mobile. Mobile was a huge window of opportunity for potential competitors, because Facebook's itself made a slow transition to mobile. Facebook staved this off by identifying and acquiring threats (Instagram, WhatsApp, and a bid for Snapchat).
Its biggest threat remains the emergence of a new platform. I imagine that's why Zuckerberg made such a big gamble on VR; if VR could possibly be the next platform, he felt he needed to own it. It's starting to look like consumer demand for VR was way oversold, but maybe an ounce of prevention is worth a pound of cure.
Another possibility is people will simply gradually lose interest in Facebook/Instagram. I deleted FB years ago and can't imagine going back, but apparently I'm not part of a bigger trend in this regard.