The Leader Is Great, Right?
If employees like and trust management, a top-down internal community push works well.
Everything the employees already thought is confirmed. The CEO wants to hear what we think, help us collaborate better, and build relationships between us - what a genius!
You can start at the top, have the CEO announce it, encourage them to participate, and watch everyone else follow.
If employees don't like and trust the management, a top-down community doesn't work well. It's seen as another cynical ploy. Doesn't the CEO know we don't have time to participate in a fanciful community? How's this going to help us sell more of our widgets?
Here you need to start at the bottom. Find the people who share a common connection. Build something just for them and gradually expand.
(aside, GlassDoor is a good guide here).