Simply type "@" in your Facebook or Twitter message, and you will get a list of Facebook Pages and Twitter handles to pick.
Unfortunately, the support of this functionality is dictated by the social media destinations' APIs, and may be revoked at any time, outside of our control. If you find this is no longer working for you, we would recommend you publish your content to your destination's CMS and edit the post in their UI before publishing to leverage the tagging functionality within their systems.