I hear this a lot, that people move platforms "all the time", but it's not really true as it relates to Facebook. Before Facebook, the vast majority of users on social media (MySpace), were teenagers and young adults. Social media only exploded to your parents and grand parents and everyone else with Facebook. It's much harder getting those folks to switch, and the network effect is that much stronger with the ~30x larger user base. Switching off something like Alta Vista to Google isn't even comparable - there are basically no switching costs in that case.
I'm not saying it's impossible, but the stickiness and network effects of Facebook are pretty much unprecedented.
You need to factor in time. As an example Microsoft isn't treated the same way it was years ago and IBM isn't the same as before Microsoft.
I'm not suggesting FB will become obsolete but people will use other services if they see value elsewhere. If that weren't the case then WhatsApp/Instagram wouldn't have grown so fast. Network-effects does not mean something is immune from disruption.
I'm not saying it's impossible, but the stickiness and network effects of Facebook are pretty much unprecedented.