You can subscribe to bot and user message text events.

Use-cases:

creating your own custom app integration
testing and other advanced use-cases

{
    "user_id": "Uxxxxxxxxxx",
    "sender": "user",
    "timestamp": 1487101585.943187,
    "text": "hi",
    "type": "text",
    "bot_id": "BJty7evfw2b"
}

{
    "user_id": "Uxxxxxxxxxx",
    "sender": "bot",
    "timestamp": 1487101587.398373,
    "text": "Hello, World!",
    "type": "text",
    "bot_id": "BJty7evfw2b"
}

📘
User initiated messages
If you would like to also receive user-initiated messages, you'll have to enable that setting in your webhook. In the case of a user initiated message sender will come in as user rather than bot.

854 — Select "Send user initiated messages?" to receive user messages as well.

📘
Subscribe to all messaging channels
If you're a Pro customer you can subscribe to message events from all channels including Facebook Messenger, Meya Web, Twitter DM, etc. This can be useful for archiving transcripts in real-time.

768 — Subscribe to all channels by turning this on.