Facebook is looking to borrow from rival social network Twitter by incorporating use of the hashtag, sources say. Seen as a sign of heated competition for mobile advertising, Facebook is experimenting with the idea that clicking on a hashtag would pull up all posts about similar topics or events "so it can quickly index conversations around trending topics and build those conversations up, giving users more reason to stay logged in and see more ads," write Evelyn M. Rusli and Shira Ovide.