Would they really arrest people just for chatting about stuff? I cannot see an investigation building a good case without material corroborating evidence, such

Would they really arrest people just for chatting about stuff? I cannot see an investigation building a good case without material corroborating evidence, such as weapons purchases, criminal profiles, movement of vehicles, packages, etc. etc. If arrests were made based solely on chats, how many kids running Call of Duty are going to get nailed, a la WarGames: The Dead Code?