Very interesting question. Cross-posting on Reddit comes to my mind. We can see Reddit as one big tree where the root node is Reddit. Directly connected are al

Very interesting question. Cross-posting on Reddit comes to my mind. We can see Reddit as one big tree where the root node is Reddit. Directly connected are all subreddits, which again act as trees to connect all starting posts in one subreddit. Real cross-posts could be modeled with a graph, where one post belongs to multiple subreddits.