My aunt always posts depressing Facebook statuses so I finally called her out about it.
I'm not sure I understand. Why did you call her out for that?
She's bringing everyone down.
You should be nice to your aunt, not talk negatively about her on social media. It's impolite to call out family members on social media, especially when they aren't saying anything particularly bad.
I need to be honest if I want her to be happy.
I don't think you calling her out is going to make her happy. If anything, it might make her upset. If you want to cheer her up you should try to have a nice conversation with her, not say impolite things.