I'm sorry if you have answered this before, but has your ex T ever indicated to you that she is interested in staying connected now or in the future. I've heard of Ts and clients staying in touch after terminating if they have the desire. Not in a social friendship usually but through an occasional email or holiday card. In this instance nothing is going to happen if they don't wait a set amount of time.

If you contact her I would through a message, though she may not get it if her security is tight. As someone else noted, 400 friends is not abnormally high and an intelligent T is very unlikely to have any ex clients as FB friends. If you saw her once or twice and terminated that might not be an issue, but for a T you saw regularly it is. Social media is risky and just contact in that forum is just not smart. Just remember that so you are not hurt if you hear nothing back from her.