One way to do this is to create a Page with an inline email form on it. You can then share that link on Facebook or other.
Topic summary
Merchants using Shopify Email want a straightforward way for Facebook users to subscribe via a link or embedded form tied to their Shopify newsletter.
Multiple participants report the same need and no clear built-in integration. One person notes support pointed them to customer accounts, which doesn’t solve newsletter signups.
Most actionable update: create a Shopify Page with an inline email signup form and share that page’s link on Facebook (and other channels) to capture subscribers.
There’s unverified speculation that Facebook may discourage native newsletter signups to keep users on its platform, but no evidence is provided.
Status: no confirmed native Facebook subscribe method surfaced. Practical next step is the Shopify Page + inline form link workaround. The thread remains open without a definitive solution.