The Official Facebook Chat Plugin的作者是Facebook,目前有70,000+个有效安装。已经Tested with 5.4.1版本的wordpress。
With one click, you can add the Facebook Chat Plugin to your website, enabling customers to message you while browsing your website. To see and reply to those messages, simply use the same messaging tools you use for your Facebook messaging, on desktop at, Facebook Page Manager App (available on iOS and Android), or by adding your page account to Messenger. It’s free, easy to install and comes with a user interface your customers are already familiar with.
Key features of the Chat Plugin:

No disconnection: Using the plugin creates a long-lived thread between you and your customers in Messenger. Customers can start a chat on your website and carry on in their mobile device with the Messenger app.
Chat transcripts are automatically created in the customer’s Messenger account
The familiar, modern interface of Messenger builds trust and encourages your customers to engage with you
You can use the same Inboxes on desktop and mobile that you use to manage your Facebook page messaging
You can set up hours of availability, auto replies and FAQ to serve customers when you’re not available