Will any big social media platforms die in the 20's?

I think this is the new norm. Myspace was significant for it's time, however it was at the peak of it's powers before smartphones had really taken off and internet access wasn't as reliable. It was more susceptible to being replaced by something bigger and better (i.e Facebook). YouTube most likely would have suffered from the same fate if a more advanced video-sharing site had emerged around the same time that smartphone adoption rates began to increase, IMO.

The one thing that really works in the favour of the current big social media platforms is that technology has developed in a way which has allowed them to remain viable for many years to come. Due to smartphones themselves becoming more entrenched into our day to day lives, the big social media platforms that exist today have essentially morphed themselves into our "address books", like you said. Going on Facebook is basically like the equivalent of ringing someone over the phone; it's the main way of getting in contact with our family and friends. It can be hard to change people's habits and because these sites have taken on the roles that they have, the chances of them ever being replaced by other social media sites are super slim. They will always be around, even if some of the sites may experience a drop in popularity.

Snapchat for sure and I joined way back in 2013. I know for a fact that neither me or my snapchat friends dont use it nearly as much as when it was at its peak at 2015-2017. It seems as though adding streaks which they thought would encourage people to use the app to only do the opposite. People would send snaps of stuff all the time like whether it was funny shit or just talking but now I only use it once in the morning to send my streaks and thats it, ive lost most of my streaks now anyway so theres no point.

A lot of the major players have successfully adapted to changes in technological culture, so I have my doubts any of them will collapse quite as spectacularly as MySpace did at the start of the 2010s.

That said, if there's any major site I could see struggling down the road, it would be YouTube, since that website's reputation just keeps getting worse and worse every year. It has become so synonymous with advertisements and ridiculous copyright rules that its novelty is starting to really fade away compared to younger video services. It's perhaps a sign of the times that YouTube Rewind 2018 is now way more disliked than "Baby". YouTube's spinoff services have failed. As streaming services like Netflix, Hulu, and Amazon Video continue to mature, they will likely find a way to better reach the crude, DIY crowd who have kept YouTube afloat for so long. I myself still use YouTube regularly because I prefer the flexibility and diversity of content over streaming services with such limited and ever-changing catalogues, but I could absolutely see the tables turning down the road with how things have been going.

It's pretty noteworthy to me that despite having existed for 14 years, YouTube doesn't really have many homegrown celebrities, or at least not many who still matter. Felix Kjellberg is the site's biggest star, but he can't infiltrate other services due to being a racist prick who can't adapt to the rest of the media landscape. People who built careers off of YouTube are going to increasingly either migrate to Patreon or start recording podcasts.