This has changed since it was first documented. The old approach, embedding a Facebook Page Plugin live chat widget that required at least 2,000 page followers, has been replaced by a Facebook Messenger channel integration, with no follower count requirement at all.
Rather than an embedded chat widget on your Facebook Page, followers now message you directly through Facebook Messenger itself, and those conversations, along with any media shared, appear in your SalesIQ operator dashboard alongside your website chats. All chats and media are also stored in Facebook Messenger itself.
If an operator doesn't attend a chat, it moves to the Missed Chats section. You can set a time limit, from 15 minutes up to a month, after which a missed chat automatically converts to a closed chat. Operators must respond before this limit, since they can't reply to a chat once it's closed. Separately, you can set a limit of up to 24 hours to automatically close chats that connect to an operator but then go idle with no further messages.