The @ Sign:
Facebook - Tagging --> On Facebook when typing in your status @username where username is someone you know - as you type @, no space and then the first character, your friends names that match will start to populate as a drop down. As you continue to type and the auto complete finds the friend in question, just mouse down and click the appropriate name. The "@" symbol will not be displayed in the published status update or post after you've added your tags. Friends you tag in your status updates will receive a notification and a Wall post linking them to your post. They also will have the option to remove tags of themselves from your posts.
Twitter - Reply --> The @ sign is for replying to a tweet. If you receive a tweet from someone you are following you can reply to them by typing @username comment. No space between @ and username. An @reply is "heard" by followers of both you and who you replied to. If you want your reply to be private then that would be a DM (direct message) - from your SMS client - d username - yes, there is a space between the d and the username.
The # Sign:
Definition: The # symbol, called a hashtag, is used to mark keywords or topics in a Tweet. It was created organically by Twitter users as a way to categorize messages. Here are some general guidelines courtesy of Twitter Help:
- People use the hashtag symbol # before relevant keywords or phrases (no spaces) in their Tweet to categorize those Tweets and help them show more easily in Twitter Search.
- Clicking on a hashtagged word in any message shows you all other Tweets in that category.
- Hashtags can occur anywhere in the Tweet – at the beginning, middle, or end.
- Hashtagged words that become very popular are often Trending Topics.
I hope that helps!