That was a pretty poor reason. (SPOILERS)

There's a tiny clue in this film that might make Laura more sympathetic and give you more understanding of why she killed herself. It's blink and you miss it. I wish they had elaborated on it more in the film.
When the group first starts to get scared about Laura's account being in their chat, Blaire messages Mitch and Mitch reminds her today's the anniversary of Laura's suicide. He offhandedly writes that Ken was right in saying that Laura was a bully. Blaire attempts to defend her. We see her about to send in "She had an uncle who was..." but then she decides against sending that so she types "She had some family stuff." but doesn't send that either, finally settling on "She had other issues."
So from what I gather, Laura was being abused by a family member (very possibly sexually), which lead her to becoming a bully. It's classic. The biggest bully in school probably has the most heinous stuff happening to them at home. But only Blaire knew that...and yet she STILL shot the "Leaky Laura" video. The video just seemed like it was the last straw for Laura. She's getting hurt at home, now she's being bullied at school, there's no safe place to go, so she decides to kill herself. After her death, our cast goes out of their way to set up secondary YouTube accounts to post hateful comments about Laura on the video (what she showed Blaire.)
So I don't think Laura came back just to get revenge on the video. She did it because her spirit had grown vengeful due to the abuse at the hands of that uncle (who probably also died the same night everyone else did.) So she decided to become judge, jury and executioner for a group of people who all had hidden terrible secrets from one another. She made them all feel the way she felt before coercing them into suicide. Blaire committed the ultimate sin (filming the Leaky Laura video when Laura was supposed to be her best friend), so she presumably suffered the worst fate. Blaire wasn't forced to kill herself, she was murdered, thus completing Laura's vengeance.
I know there was no other way to incorporate that into the film, make the characters realize the truth about Laura's bullying ways, but I wish they had tried. That would have made them feel even WORSE about themselves before killing themselves.
Btw, I'm not condoning Laura's bullying. Or any of the other characters' bullying. It's never right to be a bully, no matter what. But if Laura's getting abused, she is fated to be a bully because she obviously had nobody else to turn to in order to help her. Like you said, just as petty teenage struggles like unflattering Facebook photos are a part of life, so is the abuse to bullying equation. If you're a young person who's being abused, sexually or physically, your natural instinct, if you have nothing else, no support system, is to take it out on the people around you. Thus her reputation, thus "Haha good girl Val's gonna beat my ass," thus Leaky Laura, thus her suicide, thus her bloody revenge.

Considering how they're teens, particularly teens who are heavily into social media, this really can affect people. People often say to just delete comments, unplug your electronic devices, and so forth. It doesn't really stop there. It continues in person with taunting, nasty comments, sometimes social isolation, physical violence, and can lead to self-harm or even suicide in Laura's case. While I don't agree with killing yourself over something like this, I can see how and why this would affect someone. Adolescents are some crazy bastards, and they're even worse in this current generation. Hell, I'm only 21 and recognize how other people in my age group behave online and escalate things in person.

Anyway, I feel sympathy for Laura in a way. At the same time, I feel like hurting one of them would have been much more effective but I'm not the writer of this movie.

A bit of the old ultraviolence...

I always felt the point was that Laura wasn't justified for killing her homies over busting her balls. That they might have done some bad things, but that they were far from deserving death.

That her wanting to, adn then actually following through on a plan to kill them all was because she was either warped in undeath, a demon impersonating her, or much worse than they themselves suggested.

I really really liked it.

So... I'm gonna be blunt

Making a comparison with the J-horror, I think that's the point of these kind of movies: Sometimes the actions of a revengeful ghost are too hard to the victims. They even kill innocent people in the process. IE The Ring (the japanese)

This film could be classified as an Onryo film:
en.wikipedia.org/wiki/Onry%C5%8D

I kind of agree. And about the fact that she killed all the other kids, in my opinion, this was all apart of her plan. She kills all of Blaire's friends in front of her, posts the video of Blaire recording the Leaky Laura thing, and let her live with the fact that this is all on her for not fessing up at the very beginning.

I think the girl was abused by her uncle as a child or something.
Unless she was lying, or the Blair gal was trying to defend her and then regrets,

Anyhow, I know people who have been exposed on line while drunk and they have laughed, lived through that or become sober. Once I saw a terrible video I didn't wanted to see, a viral thing of a drunken girl being abused by a guy, I wanted to call the police but I where I live you can't report that kind of stuff unless you go to the police station, and I think eventually the people who uploaded the stuff were charged and the guy was the girl's boyfriend and both were drunk at a College party... and the girl of course didn't kill herself.

Please excuse my terrible redaction, english is not my native language