In an ideal world, maybe (although in an ideal world laws would permit more leeway for ethical action by normal citizens, and there would be far better means of psychologically evaluating people and whether their intentions are genuinely from rational ethical reasoning or from other crap...). But there's ideal, and there's reality.

Depending on what/who you're dealing with and who your local authorities are, it might not be possible to do so. In the examples I listed for example, police wouldn't get involved unless there was serious evidence already.

Especially in the cases of rape, going through the official legal channels (even more so for teenagers and people still in school otherwise) typically makes things worse, both for the victim's recovery psychologically, for the victim's reputation, and so on.

Again, not exactly true. If it was, there wouldn't be unsolved crimes and plenty of criminals who go uncaught for as long as they do.

And keep in mind that the above examples aren't serious criminals with resources and weapons. They're the kind of psychologically and physically devastating crime that people in their late teens and early twenties commit against each other, mostly. Serious criminals, the ones hardened by experience, I suspect, don't discuss their activities over facebook, on average.

Naturally, the more hardened the criminals you're dealing with, the greater the risk of death or serious injury if you get involved. Same with whether or not you end up charged/persecuted yourself (law enforcement agencies typically look very harshly upon vigilantism). But sometimes situations come up where you can't expect authorities to reliably act, and then it's down to whether the individual in question believes the ethical need to help others is great enough to warrant taking those risks.