I am a sixteen year old teenage girl who recently signed up for teenink, so this review is from the perspective of a child who does use teenink. I started writing seriously from tumblr, and I have to say tumblr's writing community and such is much more welcoming than teenink's writing community, in my personal experience.
I wanted to sign up for teenink because I thought it would be "more legitimate" in case I wanted to showcase my writing in college apps, as saying that I have a "tumblr writing blog" does not seem much. I gained certain amount of fame and praise from other tumblr followers, and followed people on tumblr who had really good writing. Unfortunately for me, I found that submitting my writing articles take a while, and once they are submitted online, rarely do they ever get commented. I think this is because not many articles are commented on, and are featured on the certain sections on teenink. My 3 articles so far have 0 comments and 0 ratings. Articles that are featured in magazines or are extremely creative (in some ways) are the ones that get most of the feedback.
In these cases, I'm extremely disappointed and am probably going to use tumblr more for writing as it has much more creativity in it. I do not recommend using teenink only for writing purposes, because I personally believe there are better writing communities out there where people do appreciate each individual writer.
Other than that, I do find the safety/privacy of teenink secure, but people may be a little rude if you go on the forums (although I have not seen this much, but the debates on religious topics can get feisty).