It turns out that in SMF 2.1 (our current version of SMF) the option for users to opt-in to receive email from other members have been removed.
However, there is a mechanism by which we can add profile fields for members to provide links to their other social media accounts (Reddit, Twitter, Facebook, Instagram, etc.)
If some members are interested in this, please put which external social media accounts you'd want to link and I'll see about adding profile fields for those.