[Full disclosure: this was written by a self-loathing "millennial."] A millennial probably served you your coffee today, their tattoo sleeve begrudgingly reaching out and saying your name out loud without making full eye contact (it's their "day job.") Yes, okay, it is incredibly easy to pick on millennials as a group because, well, for one they're a huge group. They span people born in different decades. If you're about 18 - 34 right now you are technically a "millennial." That is a huge range leaving absolutely no room for harmony among older millennials, younger millennials, Gen X, Gen Y, and Baby Boomers to see a common ground. If you look around online, you can find a treasure trove of image macros where older folks (people in their late 30s and up) really dig into millennials. Honestly, you don't really have to dig at all to find millennial jokes, memes, and insults. (But it's harder to find stuff that's actually funny.)
It's the older folk who thrive on stuff like minion memes, reaction memes, and blanket statements made about an entire age group, so you can see why you've probably seen these crossing your Facebook feed. It's either your parents or their friends (whom you only see on some holidays and send you birthday cards) who post these memes. It's usually never a millennial, not even sarcastically. This is most likely because they don't really like the word "millennial" and it's kind of hard to go to bat for a team you don't really identify with.
So millennials remain the doormat of the Internet. Vote on what you hate most about them along with the image macros that you can't help but find kind of funny.