My own purely anecdotal findings:
1. I quit fb many years ago. In my case, using it made me anxious so getting back on it would have been the challenge instead. The main drawback is that you lose out on some of the organic meetings that strengthen relationships. It's not that people forget about you, but that by simple friction you will not participate in certain interactions. This will be generational as the youngest users will be on another service and older users will have networks that aren't as influenced by fb. Note that fb is still extremely popular even among the 2. I eventually caved in and installed fb messenger without linking it to my account, which I use for certain group chats. I don't regret this usage at all, as it's only intentional communications and there is none of the browsing aspect that makes fb notorious. The other advantage is that I no longer have to explain to new people why I'm not on fb, I just add them to messenger. Many years ago, people would actually be insulted if I told them that as they would interpret it as me not wanting to connect with them further. This was particularly true in places outside of Europe and the US where fb pretty much was the internet for people.
3. I had a negative impression of Instagram from the get go. I use it to look at the work of artists and photographers, but exclusively on the laptop. In this specific use case, it's pretty great. The purely social or cultural aspect of it (by that I mean the culture and behavior that is a consequence of the app itself) is absolute cancer in my opinion. Almost like a mental illness and conformity factory unless you already have high social status or some instagrammable quality, in which case you will get much out of it I suppose.
4. Of course, it's one of the oldest traditions in the world for older generations to completely misunderstand the young. I do not envy people growing up today however. The fear of being filmed doing dumb stuff or the constant pressure is no joke.