I worked with big international brands, I've done keynotes all over europe and I write books about what I've learned.
I would start with creating a landing page that are talking directly to the target group. Get the lecturer to record a video talking to them and teasing the what they would learn.
Then I would use Facebook and LinkedIN ads targeting my dream customers. And of course a/b test all the way.
I would say that a good way is to value it by reach and engagement, also I would do some tests on how many of the people that see the link-posts click them.
Number of fans has no value, but engagement and being able to move traffic outside Facebook to your page has great value.