Facebook has announced a new element of its login with Facebook feature that will enable brands to build direct messaging connection into the login process, facilitating enhanced connection with potential customers via messaging response.

As you can see in the above sequence, now, when users go to sign-in to your website via Facebook login, businesses will also be able to add a Messenger connection opt-in as well, providing another way to maintain customer connection, and amplify promotions with interested users.
As explained by Facebook:
"Login Connect with Messenger [is] a new feature that allows people to opt into communicating with a business through the Messenger Platform right from the Facebook Login flow. With this new feature, we’re helping businesses cultivate better relationships with their customers by providing stronger customer care and more tailored engagement through the Messenger Platform."
That could play a big role in maximizing direct connection, which is especially important given Apple's recent ATT update, which now prompts iOS users in regards to data tracking in all apps. As more people choose to opt out of such, that will reduce your insight for ad targeting - which could make direct connection tools like this even more valuable as a means to ensure you're able to get your promotions and awareness efforts in front of the right people, at the right time.
Definitely, Facebook's data suggests this could be a valuable consideration:
"We’ve seen more than 70% of users opt in to messaging with businesses that enable Login Connect with Messenger. This creates a vital source of new customer conversations for businesses looking to double down on customer engagement through the Messenger Platform."
Having direct connection to people's inboxes could end up being significantly more valuable than the data insight you may or may not be able to access via general Facebook login, with response rates for messages much higher than social ads, and other outreach options.
Of course, you do have to be mindful of that connection, and the privileged access it provides. Spamming people's inboxes is also far more intrusive than social ads, so this can easily be abused. But used in moderation, and with user interests in mind, ideally through proper segmentation, this can be a highly valuable process, worth considering in your connection flow.
Facebook's new Login Connect with Messenger is currently in a closed beta, with Facebook planning to roll the option out more broadly in the coming months.