Add facebook to supported social media platforms list

[arkid15r]

(Anyways, we'd be grateful if you allow a facebook page to be specified in social media since this is where our members are mainly engaging for now. Considering to move to bluesky sometime, though)

OWASP/www-chapter-saitama#106 (comment)

[swankystark]

Hi! I'd be happy to work on this issue.

I can add Facebook as a supported social media platform and update the relevant
validation/configuration accordingly.

Could you please assign this issue to me?
Thanks!

[arkid15r]

This was addressed in #599