It depends on the child. Thinking back to my childhood, my parents never knew where I was. I bet they wish we had cell phones back in the day instead of calling around the entire city looking for me lol.
It's a great convenience for the safety of our kids these days. And, of course, it has it's disadvantages. I find though, if you raise your children with respect and trust and arm them with the tools to handle the pitfalls of social media etc.. then what do we really have to worry about? I know my kids have always been mature enough to handle it.. they know about online bullying, preditors, respecting others, privacy etc etc.. because I took the time. If someone doesn't raise a child to be responsible and just throws a cell phone their way, then no.. I don't agree with that. But it's still a great way to stay connected with the child when they aren't home, so it's not so black and white.