Someone I'm Facebook friends with always posts about how her husband wont let her have certain friends, he won't let her get a drivers license, or let people watch their kids that HE doesn't approve of. How do I tell her this is abuse?
Intimate partner violence is far more complex and far more common than we think. There are professionals who would be far better at addressing this topic than I would. Willow for example here in Rochester, and the National Coalition Against Domestic Violence nationally. Disclaimer stated, I've studied the subject a bit and have some thoughts.
First, it is important to address your own expectations. You aren't going to "save" this person, especially as they may not want to be saved. Getting into a relationship is easy. Getting into a bad relationship is ABSURDLY easy. And once you're in, you're invested. I've always understood this best through an economics term (wait, don't close this tab yet!) called the sunk cost fallacy. In this scenario someone who has already invested heavily in a business will continue to do so long after the business starts to lose money. The rational thing in these situations would be to cut the losses and move on. But we're humans and not emotionless robots, so we continue to invest in the hope that things will turn around. We chase good money after bad, rather than admit that we wasted our money. In relationships we are afraid to admit that we wasted our time, our energy, and our emotional reserves. The fear of admitting defeat and starting over are often greater than the pain we are going through. This is especially true the less overt the abuse is.
Next, let's focus on what your friend is doing. We often use Facebook as a place to vent and receive reassuring words from friends, acquaintances, colleagues, and sometimes that creepy guy who just wanted to look at your pictures. It may not be the best use of the platform, but it's a human one. Your friend may be stuck in a life that she can't get out of, and venting just may be how she handles it. It's natural to think less of your problems once you've got them off your chest. Because, well, objects weigh less on you when they aren't sitting on your chest. Temporarily free of that burden, your friend may react harshly to you criticizing her husband. Her Facebook posts may be the entirety of the bad he does, and with that subtracted she may see only the good left in him and defend him.
I know you are eager to jump on in and tell her that she is being abused. That is natural to anyone with a good heart. You may also want to suggest that she leave him, another natural impulse. But with such a sensitive topic we have to think how our words are being perceived, and how to make the best use of them. Instead of doling out one-size-fits-all advice we need to understand your friend's circumstances. This means finding out more information than comes from her Facebook posts. This means being a friend.
If there is a greater social need than to be heard and be understood I cannot name it. A genuine "how are you?" given not out of supermarket checkout line formality but with honest-to-goodness curiosity is like presenting a gift. It is also an excellent place to start with your friend, and just about the best use of Facebook Messenger. From there, using active listening skills and asking follow up questions you can build trust and rapport with your friend.
Something else you could say:
"I see from your posts you've been having a rough time." Simple really, note that it isn't even a question. I didn't add "are you ok?", which invites a conversation-stopping yes or no answer. Rather it is an invitation for your friend to expand on what has been troubling them. Going down this path you will hopefully find out more and more about her situation. Judging by her use of Facebook posts, she may be more than eager to share.
As this process continues you may face the impulse to jump in with an opinion. It's important to restrain yourself a bit. I suggest thinking about how much you enjoy when people who don't know you well tell you you are doing everything wrong and should change your lifestyle radically. Yeah, don't like that much, do you? Instead people need to come to conclusions on their own. In my personal experience this is how I've gotten the most out of therapists. I yap and yap about what is bugging me, they wait for a lull and ask a poignant question, I consider how what they've asked fits in with my line of thinking, and yap some more until I reach some breakthrough. Then I go home thinking that I solved my problem on my own and that therapy is a waste of money.
Her having a breakthrough and taking steps to regain control of her life is not going to happen immediately. Even with your best efforts it may not happen at all. But to me, a life not spent being there for others is a life wasted. So talk to your friend. Not as someone playing a Hollywood superhero, that is fiction. Nor as someone who knows what's best for everyone, that is annoying. But as a human on equal terms. You may even gain an offline friend out of it.