Hi Stephanie,
1. Yes, I would suggest you respond to the accountant's action if he has already initiated action and served you with a writ. Otherwise, you risk having default judgment against you.
2. Truth is a complete defence to defamation. Can you show support that the statements you made about the accountant was true? Further, if the statements were opinions as opposed to allegations of fact, then that is also a defence to defamation. Finally, you can argue that there was no, and will unlikely be, significant harm or damage to the accountant's reputation because the only people who saw the statement are those in your private Facebook page. This will depend on how many friends and subscribers you have, who can view your posts, what type of people these friends/subscribers are, what you wrote etc.