StephanN83 gave a really good definition of the available network types. I would like to add that the uni-modal networks are those networks who contain only one type of node (i.e user-user and post-post networks) and bimodal networks are those networks who
contain two types of nodes (user-post network).
The user-user network contains users as nodes and there will exist and edge between them if they have commented in/liked the same page post.
The post-post network contain only post nodes and there will exist an edge between them if they share one or more commenters/likers
The user-post network contains user nodes and post nodes. There will be and edge between a user and a post if the user has commented/liked the post
I hope this helps