Sam is the classic combo of tough-as-nails and deeply wounded. She's an ex-addict with a heart of gold, a damsel in distress with a "Go to hell" attitude.
And it's all her dad's fault.
Let's look at her diagnosis of her father/daughter relationship real quick:
MIKE: What did he do to you, anyway?
SAM: He was never around.
MIKE: So what? That was it?"
SAM: No, it was how he tried to make up for it by constantly trying to convince me that I was special.
On the surface, that doesn't actually sound too bad. Dad's not around (because he's busy being a Hollywood bigshot) and then, when he is, he overdoes it with the fatherly love. But if we scratch the surface of this family dysfunction, we see how it affects almost everything Sam does.
One of the ways this hot and cold relationship with her dad may have affected Sam is that it made her fade into obscurity. She doesn't like being in the spotlight because, when it came to her dad's affection, the spotlight made her think of the neglect that came before it.
That's why she shies away from being too present, and why Mike is prompted to say things like:
MIKE: You make yourself invisible, hiding behind this fragile little f***up routine.
And we might even be able to draw a line between the fact that Sam had a past as an addict and the fact that her father's interest in her yo-yo'ed. Drug addiction let her forget herself; she didn't have to wonder if she was either a) worthy of abandonment or b) worthy of lavish praise.
In fact, in her biggest monologue, Sam rails against her dear ol' dad and pretty much delivers a treatise on invisibility:
SAM: […] let's face it dad, you are not doing this for the sake of art, you are doing this because you want to feel relevant again. Well, guess what, there is an entire world out there where people fight to be relevant every single day, and you act like it doesn't exist. Things are happening in a place that you ignore, a place that, by the way, has already forgotten about you. I mean, who the f*** are you? You hate bloggers, you mock Twitter, you don't even have a Facebook page. You're the one who doesn't exist. You're doing this because you're scared to death, like the rest of us, that you don't matter and, you know what, you're right. You don't. It's not important. You're not important, okay. You're not important. Get used to it.
Riggan might have written/directed/starred in a play because he's scared to death that he doesn't matter, but Sam seems to be "making [herself] invisible" for the exact same reason.
Of course, because Sam's a dynamic character, she is also the single most visible person in the movie…at least on social media. She understands how important these media are in the 21st century and how, without a presence on at least Facebook, you "don't exist."
And in some ways she's the person who brings Riggan back into relevance and existence, first by alerting him to the fact that the video of him running around Times Square in his undies is actually pretty powerful PR, and then by making him a Twitter account. She's realized just how scared her father is of fading into oblivion, and so she's made sure he can be visible to everyone…not just the kind of people who will read Tabitha's theater reviews.
But what do you think? Is this an actual act of kindness, or a kind of revenge against a guy who was "constantly trying to convince [her] that [she] was special"?
We'll never know.