Twitter users often message each other. This can be done by tweeting "@username", or via direct message. However, that does not mean that you will receive a message; ultimately, the answer to your question is just "possibly"
Yes, that is what it is for.
Click the button that is down to your right to send a message on facebook to someone that is online!
your phone
Yes but they may write back an unhappy message.
You click on the reply button in the top righthand corner of their message.
A 'private' message that doesn't appear on any timelines. You can only direct message someone who is following you.
You can't write a message on someone else's message board if you are blocked. Though, if you did manage to write a message on someone's board, there is no way to know if they read it or received it or not.
no
You inbox someone by messaging them. If you inbox someone his/her message will go to his/her inbox.
Depending on the error, it could either mean that someone posted before you got a chance to, or possibly, the message board could have been protected. If you get a yellow message, and there is no box to type, then it means that the message board was protected. If you are able to click the post button, but then it pops up a screen asking you to edit the board, then it means that someone posted before you.
The person you're trying to send a message to has to be following you.
Either the bottle breaks on the shore and the message is lost or someone finds the bottle, opens it and reads the message.