Your browser now communicates with Facebook using a secure connection by default. Two years back Facebook has introduced option for users to connect to their site using secure connection, now they’ve enabled HTTPS by default for all users. With this, communication between your browser and Facebook servers will be more secure. According to Facebook’s Software engineer Scott Rentro in a blog post more than third of users enabled this feature for their accounts.
Facebook confirms all traffic to facebook website and 80% traffic to m.facebook.com uses secure connection, Facebook apps for Android and iOS are already using https since long time for now.
You can read more details on this here.