OP - same situation here, my DD12 also shares an account with a friend but shows only random 'aesthetic' videos (as they call them), and not images of themselves. Her other account is private, and I subscribe to both on my account (which I had to create for monitoring).
I share the view that prohibiting access, when all the other children are doing it, will be unnecessarily problematic. I would rather she learn these new social media skills whilst still relatively innocent, and use whatever safeguards there are available alongside a healthy awareness of the dangers. Hopefully she will get into to acceptable habits. 
We were lucky not to have all of this around when we were children, but our DC are growing up with this and my fear was that they don't keep up with the technology or risk awareness.
For our family, I have made it plain to my DC that if they have access to this stuff then I have full access to their devices at all times. I need them to know that I trust them to monitor what is happening and to tell me of anything uncomfortable that happens (which so far seems to be juvenile friendship issues/minor bullying issues which have been called out). I appreciate that they will have secrets (didn't we all), and that they are at risk of seeing things they shouldn't, but to my mind this is an essential life skill best learned as soon as they're ready.
As for persuading them that number of likes is not a good thing, I just keep banging on that same drum in the hope that it eventually gets through that their self-worth is not determined by something that isn't real.
Good luck.