Type in @username of the recipient to write your message. There is a 333 character limit.
All messages are public to everyone.
Use the comment and share features  to highlight topics that are interesting and important to you.
Use #hashtag to have discussions around a specific topic. Users create hashtags by placing the hash character (or number sign) in front of a word or unspaced phrase.
Searching for that hashtag will then present each message that has been tagged with it in the feed.