I myself have joined many such pages and groups. So, I can share what made me participate, like and visit them often.
First of all it has to invoke public curiosity, interest and also their spirit of looking for like minded people.
For instance, I have joined pages for content writing, philosophy, psychology and my favorite actors and singers.
The same way, there are people, subjects, issues and events that automatically draw people's attention.
Two such persons whom people cannot avoid if they come across are like Michael Jackson and say Mr.Bean.
Another subject which people generally don't ignore is "Environment", "Jobs" and "shopping".
So, you have to make a good guess work, observation and analyze. The next stage would be picking something that matches your interest too. And the continuing thing would be promoting them with latest posts every now and then.