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This is Facebook's official Messenger app. If you're already signed in, you'll see a list of recent Messenger conversations. Otherwise, click Continue as (your name) or enter your login information as prompted. It's at the top-left corner of the screen. You'll now see a list of your Messenger contacts who are actively online. If you only see your own name, slide its corresponding switch to the On (green) position. Your online contacts will now appear.
Point your web browser to https://www.messenger.com. Sign in to your Facebook account. Click the blue gear icon. Click Active Contacts.