I saw a tweet:
People who joined Facebook after 2017 are sus.
What does this word "sus" mean in this sentence?
This is a slang term used primarily by members of "Generation Z" (currently aged 10–25) to mean "suspicious" but also "dubious", "strange", "creepy", or "socially unacceptable" (in the sense that a "creepy person" is socially unacceptable).
In this sentence, it is saying that anyone who joined Facebook in the year 2017 or afterwards is creepy or socially unacceptable.
As CharybdeBE noted above, it's a slang term which has become popular thanks to the Among Us game, a multiplayer game where one or more of the characters are secretly evil aliens trying to kill all the other players. A large part of the gameplay involves accusing other players of being an alien, which is often done by calling them "sus".
This is a pretty good explanation:
Whether it's directly descended from the older term "suss", or if it naturally evolved independently is a good question! Personally, I'd guess it evolved independently, but I've not attempted to do any research into it, so that guess may well be... sus.
Still, I wouldn't be too surprised to find there's a student thesis or academic paper floating around which attempts to answer that question, though!
Sus (sometimes spelled "suss") is old, old slang, with two related main meanings.
The meaning that applies here is sus = suspicious/suspicion.
(It can mean "suspect" but only as an adjective meaning suspicious/dubious such as "that document is suspect, it could be a forgery", never as a noun "John is a suspect in a robbery")
In the UK during the 1980s, it was slang that someone was "arrested on suss" - meaning on suspicion of being up to no good, carrying stolen goods or drugs, going equipped for crime, being involved in a crime that happened, loitering with intent to commit a crime.... something like that.
The same word and meaning, was also used to refer to an object or action, as being suspicious, suspect, dodgy, or otherwise not to be taken at face value or given good faith. So whether a person joining a protest was a genuine protester or an infiltrator, whether something was an accident or deliberate, whether something happened in good or bad faith, whether the gift is really a gift or an enticing trap/trick...... "It's (or They're) suss" or "It's (They're) a bit suss" (meaning "suspicious/suspect") covered all these shades.
So this is the meaning of your Facebook quote. The speaker is saying that people who joined Facebook after 2017 are suspicious/suspect in some manner - less likely to be seeking social media for the usual or traditional motives, or more likely to be dubious in some other manner. They need extra scrutiny as a rule, in a way that pre-2017 Facebook users usually don't need.
(It can also be used less seriously, I'll cover that below)
The other, related meaning, is most often seen in the phrases "sus(s) out" or "sussed/sussing out" or "sussed/sussing it". Meaning roughly, to have figured out or solved a conundrum, puzzling thing, problem, or anything else.
The connection to the first meaning is that the adjective something is sus(s) becomes used as a verb "to suss", or to "suss out".
Once someone or something is "sus", the logical next step is trying to figure out whether it/they are in fact the dodgy and bad faith things that they are suspected to be. The process of working that out naturally in slang terms becomes "sussing out" or "sussing it out" and having an answer becomes "sussed (it) out".
Note that while "sus/suss" always implies suspicious/suspicion, the related verb "to suss out/sussing out" does not automatically imply suspicion. As often happens, the slang "drifts" in meaning. The verb can simply mean to solve or figure out, a thing.
OP: I got a text telling me about my tax refund, but I don't pay any tax
A: Sounds sus to me, mate
B: Yeah, definitely sus. Go check with your employer's HR team see if its legit? [=legitimate, valid]
OP: Okay, sussed it out, its a scam, my friend got the same text last week and the website's a fake.
B: Great, I'll tell Joe not to bother checking the phone number if you've sussed it already.
And some examples as a verb,where it simply means "to work out or solve a thing":
I just sussed out (worked out/solved) that crossword clue!
My dad finally sussed (worked out) that I'm an adult and can do things like visiting friends without asking permission!
(The implication being, he had to think about it to realise the speaker is an adult and therefore can do these things.....)
For example in humour:
"People who drive Honda cars are definitely suss"
(meaning roughly, in humour, the speaker is suggesting there's something wrong or dubious about the kind of person whom would choose to drive a Honda car)
OP: "I ordered chicken and pineapple on my pizza."
Friend: "Pineapple? Sounds sus to me, mate!"
(in humour, joking at their friend's choice of topping)
So your tweet could also be making a joke, that people who joined Facebook recently are probably suspicious, but may not be serious about it. We can't tell which it is, serious or joking, without more context and perhaps even then its not totally certain.