Facebook is definitely popular. From what I have seen, the people who are the most successful selling seem to have large professional networks with other trainers and horse people, and that definitely is a factor. They also tend to belong to a number of horse related groups, and advertise the horses they have for sale in the groups that are a reasonable fit for the type/breed of horse, discipline involved, and geographic area where the horse is located.
I don’t know if this helps at all, I just wanted to explain some of the factors that seem to influence how successful people are when trying to sell on Facebook.