Let me just start off with I. LOVE. TUMBLR. I believe it is a great blogging tool for teens who are apt with technology (if you know a bit of HTML code, it can be a life saver when editing your theme), and are the creative type. But honestly, please keep kids who are under 13 off of Tumblr, unless you've discussed this with your child and know they can handle it! I'm not one of those users who hates the 12 year olds because they're 12, but because some things can get well... raunchy.
There's a lot of sexual references, porn gifs, drugs, alcohol, self-hate (though this has slowly got a lot better), and of course, bad language. BUT, there is also a lot of positive messages, warm and fuzzy things, cute animals, funny posts, informative posts (I have learned so much from this website, whoever says you can't learn anything from Tumblr is wrong.) cool art, and so many more good things.
You can't always look at the bad side of Tumblr, there is a good side! And for the most part, you only see the bad side of Tumblr if you follow blogs that reblog and post those kinds of things. Honestly, kids will run into the same stuff on Facebook and other Social Networking sites by accident. You cannot shelter your kids forever, you need to talk to them about these things, it won't want to make them do those things, it will prevent them from doing it! As for online predators, there probably are a few on Tumblr, but none that i've seen. Again, TALK TO YOUR KIDS/TEENS. And please realize that not everyone that your child doesn't know is a 50 year old pedophile who's out to get them. So, Tumblr would be best for kids who know what they're getting into here. Be sure you talk to your kids and know what their maturity level is!
Now about the ask and anonymous/anon features; YOU. CAN. TURN. THEM. OFF. And Tumblr WARNS you that you can get anon hate! They tell you what you're getting into and it's your responsibility to use it wisely!
Tumblr is great because it has many media forms (text, chat, audio, video, link, photo, and quote), and is very customizable! They really just give you the URL and the blank page and you can go crazy, with a little internet search you can have a cool theme, background, music player, cursor, and all these other little things on your blog than can really make it stand out, and lets people express their creativity. And it's great for meeting other teens who love the same things your teen!
The reason why I use tumblr, is because not many people in my area really like the same things I like (unfortunately), and so I meet new friends online! It can really prove that an internet friendship can be just as real as a face to face friendship!
And i'm into Korean Pop (no, not PSY. There are more artists than PSY.), so I don't really come across the drugs, porn, and self hate at all... it depends on the blogs you follow, if you're still skeptical, take a peak at your kids dashboard, or what blogs their following.
And not to worry, since this is a blogging site, people tend to be more truthful because it's for personal use! There are probably people who lie about themselves, but that doesn't really happen as much on Tumblr as it does Facebook. And there are no viruses either, that was just a rumor to keep outsiders out.
tl;dr? Tumblr is not as bad as many people make it seem, it's actually a great website that people can get hooked on, but be sure you know how mature your child is first before letting them get an account. Predators aren't really a big problem here, but be sure to educate your kid or teen about online predators, but don't scare them into believing everyone online IS one.