Twitter has over 500 million registered users who post 340 million tweets per day, showing its widespread popularity. There is a 140 character limit for tweets, requiring users to write short paragraphs. Hashtags are used to categorize tweets by topic and Twitter displays trending topics. Direct messages can only be sent between users that follow each other, while retweets allow sharing tweets with those who don't follow the posting user. The @ sign is used to mention other users in tweets. Notifications are used to alert other users.