Shoutbox

Explanation:

If you type /ping to your contact and they respond with Pong, it means they have Messenger Plus!. If they don't, they either have it turned off or most likely don't have Plus!.

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How to stop responding to Ping? requests:

1 - Download Messenger Plus! Tweak Live

2 - Open the file, which will bring up a box. Tick agree, then Import the script.

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Alternatively you can edit the registry manually. If you haven't edited the registry before, be very careful. The above method is a lot easier.