Social accounts, such as Facebook profiles and Pages, LinkedIn profiles and Company Pages, Twitter, etc. can easily be added by going to Services at the top of your dashboard, and then Add Services.
On the right-hand side, you'll see different categories of services. Simply click on Add under the service you want to add. You will then need to enter your login details for that account, and in some instances, accept permissions so that your Sendible account has the necessary permissions to post out or manage the social account from within the dashboard.
For details on how to add a particular service, please visit this link.