Some Japanese slang resources
Never underestimate the power of learning a language through breaking it... especially if you want to immerse yourself more in contemporary Japanese culture, or want to wade through the mind-boggling minefield known as social media posts in a foreign language. Plus, slang instantly makes you look like you have a better handle on the language than you might actually have, and I'm all for sounding more adept than I actually am (笑). After all, it's not like we all speak in totally proper sentences 24/7 in our native languages! So, what harm could it do 'ya? Here's some resources for learnin yerself sum slang alongside the proper stuffs.
Here's some commonly-used phrases that Japanese high school girls use in texts: https://tokyogirlsupdate.com/high-school-text-slang-201706125529.html
On the other hand, here's some pretty gender-neutral high school slang. Scroll down to the bottom of the article for the list of words: http://www.japantimes.co.jp/life/2015/08/31/language/drop-get-high-school-slang/
There's an ad for another Japanese program in the beginning, so just scroll to the bottom again and you'll be fine. Here's some more common words that most people - not just high schoolers - can use and be "hip": http://www.fluentu.com/blog/japanese/japanese-slang-words-phrases/
Here's some common abbreviations that you'll probably mostly see online, including the iconic (w)/(笑), the slang that corrupts your vision in Nikoniko videos: https://blog.skritter.com/2014/10/acronyms-and-abbreviations-in-japanese/
(Small mentions of adult themes) Not sure what I meant by that? This article explains what I was talking about, plus more common internet terminology (Googling, etc.): http://maggiesensei.com/2010/10/14/%E3%82%B0%E3%82%B0%E3%82%8Bguguru-net-slang/
Abbreviated Japanese is usually the first thing that I think of when I think "slangy, informal Japanese." I'd recommend reading the whole thing, as this is the only article without a word list!: http://www.japantimes.co.jp/life/2009/12/23/language/yappa-abbreviated-japanese-aint-all-that-bad/
Keep in mind that I tried to keep this as SFW as I could, since I wanted to make this as inclusive to everyone on Duolingo as possible. I was going to recommend two books that I've read in the past that explore the more adult side of Japanese slang, one in a more practical way, one in an absurd and over-the-top way, but I chickened out for the sake of the purity of this discussion. If anyone's interested in those recommendations, I'll post the names and what I recommended about them as a separate comment on this discussion.
Have any more resources to share that I might have missed? Got some slang to share if you're an experienced speaker? I'd love to hear it!