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That is, make sure you're not interrupting anything. People may respond to you but not really have time for a drawn-out chat. If the person makes it clear that he doesn't want to chat now, ask if you can chat in the future. If he says no, respect his wishes. Most people are turned off by bad grammar. Also, if you're over 20, avoid using "netspeak," such as "u" for "you" or "r" for "are." If you send a couple of messages and he doesn't respond, stop trying to initiate conversation, especially if your messages are marked "read" by the messenger.
Always ask if the person has time to chat. Back off if the person doesn't want to chat. Check your grammar. Stop trying to initiate conversation if he doesn't respond.