The way we used to say some things years ago doesn't work anymore. Do you know why? Expressions tend to assume different forms and change everyday, it's part of the language. Of course, not everyone is talking using slang (it's up to you decide what to say, the mouth is yours, right?). But let me tell you some examples I recall at the moment (most of my American friends say a lot of slang (based on what I see in our daily conversations, and I'm not 100% sure about their meaning)):

1) Bae (a short form for baby/babe or "before anyone else").
2) Turnt (means you've been in a big party and drank too much (alcohol)).
3) Can't even... (Hmm... This one is kinda easy. You got so emotional (excited, angry, mad, in love) that you couldn't even... say something! Haha)
4) #TBT (post old photos in social media (Facebook, Instagram, Twitter, etc.) every Thursday (Throwback Thursday)).
5) Slay (you are the best by doing something (or at least better than him/her/them)).
6) Killin' it (you are rocking, making something to show off (and you're making success)).
7) Lame (annoying people, a shame, nobody wants to be a lame).
8) Creepy (say something perverted/acting pervertedly).
9) Hoe (absolutely a bad word for "dirty girls").
11) Dope/so sick (something good enough to make you feel great/a good mood).
12) Swag (you have style, look good, cool).
13) Shipping (basically wishing two celebs/characters being in a relation'ship' (Twitter fandom usually do it with hashtags)).
14) Mall (shopping center).
14) Squad (a group of friends).
15) Fly (feeling yourself, stylish).
16) GOAT (greatest of all time).
17) The feels (high emotions; when you get extremely emotional for anything you love the most).
18) Goals (no, I'm not talking about football. Basically, it means to wish something you don't have (like a friendship with an idol or anything else)).
19) Bye Felicia (gilrls usually say it when someone irrelevant leaves the chat/room).
20) On fleek (on point, perfect).

There's a whole world of words (www, haha) beyond these I just wrote including thousands of short forms, mainly among young people in social media. Thus,
what should be kept only on the internet is brought to real conversation. I mean, if you don't know what they mean (lol, bff, otp, stfu, af, lmfao, wtf, asap, idk, ppl, hmu, np, l8r, brb, k, u, etc.), probably you won't understand what the topic in any conversation in social media is talking about. It is what it is. Deal with it! If you know other slang, please comment!