The stream header in the news feed that lets the user sort their news feed by "Top Stories" or "Most Recent".
Also, this can be a section separator. For example, in group page, the header that says, "Pinned Post" and "Recent Posts".
Found: News feed and group with pinned posts
The "Top Stories / Most Recent text" text.
Found: News feed
- .uiStream .uiStreamHeader .uiStreamHeaderChronologicalForm
The "Pinned Post / Recent Posts" text.
Found: Group with pinned posts
In the news feed, sometimes facebook injects a story like: "Friend A and Friend B like Page X". Under that story exists a section called "Related Post" which displays a related post from Page X.
In the news feed, when you like a post by some page, sometimes Facebook will offer you to like pages they think are similar.