I appreciate you reaching out to the Community with this inquiry. I reviewed your account and I see you send out campaigns designed in our custom code editor. Currently, the Social Share feature for posting to Facebook, Twitter, and LinkedIn is not integrated with custom coded campaigns. We are collecting feedback on having it integrated under the new interface and I will make sure to track this feature in your account.
You are still able to post manually by copying the Share URL from the Email Details page of a campaign. I do apologize for any inconvenience.