There are two ways to empower people to use social media. Carrots and sticks.
At my previous company, one day you can only use social media if you have been on a two day training course on dealing with the press and you are registered as a spokesperson at HQ. OR they will fire you. The next day this policy had changed and they wanted everybody to talk on social media.
The flea in the jar - If you put a flea in a jar with a lid on, the flea will learn to jump to the lid and no further. Take the lid off, does the flea jump out of the jar? No! The behaviour is inbuilt, it carries on jumping as if the lid is there.
The same with employees. You told them not to post, or to post and follow the rules, which if you read, the best thing to do (so you don't lose your job) is not to post. You can try all kinds of things, fact based training webinars, putting objectives into performance reviews none of these change people's behaviour and habit.
If I want to lose weight and get fit, I need to change my diet and got to the gym three times a week. It's pointless putting me in the gym for two days, or showing me what a gym looks like through a webinar. Worse writing that I need to lose weight in my performance review. This does not get change.
Employees need to want to do employee advocacy. It needs to come from inside, be motivated to why this is good for them (first) and the company (second). They need to stop being scared, understand they might and can make mistakes. They need to be empowered and activated.
While I'm here, promise me you won't use employees to push corporate content. We all know nobody wants to read corporate content, employees more than anybody else. I often talk about the "dinner party rule". If you talked about your company, how great it is, how it's number one etc at a dinner party with friends. How long before somebody told you to shut up. Seconds probably. Certainly with my friends. So don't do it on social and please, please, please don't force your employees to do it.