Only a limited subset of notification events can be delivered via social media:
For anniversaries, awards and birthdays, if the referenced person has the contact flag "No social media" set, VSys won't create a social media notification even if the person meets all of the notification's other criteria. VSys also does not "tag" your volunteers in notifications which reference them, it only includes the fields that you designate in the notification (usually the person's name).
Because of the nature of the various social media sites, VSys can only post to your page or news feed. (Facebook's permissions structure prevents VSys from posting to your volunteers' pages.)