It doesn't look that way to me, in fact they all look quite vulnerable in various ways, some common and some unique. Netflix and Facebook will be the first to stumble I suspect. I can't predict the future and I'm not an active investor but for my part my retirement savings are not going to be heavily weighted in FAANG stock. I guess we'll see how the landscape looks in a decade.
I don't really see how you could say they're not stumbling. Have you looked at the stock chart recently? Have you noticed the barrage of negative press from the NYT and other sources just in the past week? Have you followed the congressional hearings and increasing calls from both left and right both in the US and Europe to regulate them? Have you seen the increasing calls for Zuckerberg to step down as chairman? Have you seen the articles reporting both the steep decline in Facebook usage among teens and the increasing evidence of the connection between social media and teen depression? Have you noticed the increasing number of people who hate Facebook the product and the company and are abandoning it? The stream of executives leaving over disagreements?
Now Facebook has risen a long way and has a lot of buffer to recover from missteps and challenges. I'm not counting them out yet or saying they're facing an existential threat soon but the shine has very clearly come off in the last couple of years. There's certainly room for debate about the magnitude of the challenges they face and how well placed they are to overcome them but I don't see how anyone can question they are already stumbling.
Look at the fundamentals [1]. Facebook (the product), Instagram, Whatsapp, and Messenger completely dominate the social web. Oh and in case you thought Snap was a competitor, realize that Snap pays Google $2B a year and Amazon $1B a year for Cloud Services. On Flat growth and relatively terrible network effects. FB doesn't have to pay anyone because it has it's own services.
No other collection of social networks or advertising networks are coming anywhere close to them. Nobody is seriously threatening their business model. The recent Frontline piece was really hard on them, but there's nothing there really tangibly. Here's the reality:
Facebook has the predominance of internet users across all platforms and still growing 11% annually.
Facebook still sells advertising that converts better than any other advertising platform.
Facebook has diversified and built information networks on par with Amazon and Google to prevent (as much as anyone has ever done) any competitor from surprising them.
The only possible thing that could happen is some government trying to shut them down. Honestly though without specific legislation against them and only them, it won't work. You can't kill the advertising model with legislation and you can't kill data collection (Like GDPR is trying to do) without it impacting all players - if anything it hurts small players more.
So no, I don't see them stumbling. In fact I don't see one example of Facebook, Amazon or Google being surprised by any challenging upstart in the last 5 years. They might have been rejected for buyouts, but they weren't surprised.