Keep your posts simple and meaningful. Remember, you’re not trying to impress anyone with your command of the English language. You’re just trying to share a message, and in doing so, pre-frame your audience’s mindset.
Keeping your post simple and meaningful will allow the reader to easily engage with your post. They can quickly read it, understand it, digest it, and apply it to their day – and then move on with your post in mind. You don’t want your post coming up in conversation later for the wrong reasons – “Oh man, I saw this post today with, like, all these giant words… who are they trying to kid?”. Nobody wants that – nobody.
Also consider what someone might search for online. Are they going to use large multisyllabic words, or short ones? I’d put my money on the latter, if I were a betting man.
Remember your audience though – there are times when the phrase or long word might be JUST the thing they need. As with everything, it’s a balance that you need to test and learn from.