With our oldest, we let her get social media (first Facebook, and later Instagram) at 13. I thought it would be smart to do it that way so we could coach her through it and help her be responsible with it. In my mind I equated it with teaching a kid how to responsibly use a firearm. Except social media is more like a drug than a firearm and there's no way for a young teen to responsibly use it. It's inherently harmful, and I think it was a major contributing factor to her anxiety/depression and other struggles.
With our second/youngest child, she still got a phone at 11, but we really locked her phone down. No social media and she has very small amounts of time she can play games on it. We've still had some issues with her viewing inappropriate videos on YouTube.
Part of my thought in choosing smart phones was that, at the time, the cheapest smartphones were about the same price as a dumb phone. Why not just get a smart phone then? It has more features after all! Stupid decision. If I had to do it all over again, I'd wait until high school to then get them a dumb phone. Wait until 16 to allow them to get a social media account, and perhaps buy them a smart phone around 16-18.