Teens typically get to a point where they feel that they are mature enough to make their own judgement calls and not risk bodily harm. The challenge is that this isn’t always the case. Fourteen is typically an age when they should start being treated in a more adult-like way.
Have you asked the niece why she has ~500 Facebook friends? Asking open-ended questions like that may help to keep her from going on the defense. You might also be surprised at what she offers as an explanation. I can think of about a half a dozen things that she might say that would be acceptable.
…I just saw your last post show up, and I think I’m missing the point ‘some of the friends list ends up talking about suicides, etc.’ You might be able to get her to whittle down her friend count, but how do you stop some on the existing list to not post topics that you approve of? Trying to protect her from postings like this isn’t limited to Facebook. If the suicide topic is a specific concern, isn’t that something to discuss one-on-one? I realize that you are in different locations, but it can be done over the phone. (Have you considered web-camming with her? It’s a step better than over the phone and it’s free.)