Certainly, your relationship with your learners is crucial for the smooth functioning of the language learning experience.  Part of this relationship requires a good balance between familiarity and formality. 

Most important is that your friendly exchanges during class time be in English and not in the mother tongue.  When the learners see that you are willing to converse with them in the language that they have signed up to study, they see you and themselves in a new light.  Their individual self-esteem and their respect for your professionalism increases with every target-language exchange!  So, allow and encourage them to build their social relationship with you in English, even though you may speak and understand their mother tongue.  

Within the constraints of your overall lesson planning and sensible time management, it is great to chat informally with learners so as to expose them to everyday, colloquial English.  Relationships with learners can be enhanced with casual, non-threatening exchanges  and the wonderful result is that as learners' confidence and fluency grow, they start to consider themselves actual users of English.

The necessary formality is established through your demeanor, presence and procedure while working with learners.  A teacher earns much respect from demonstrating a commitment to using lesson time wisely; she sends the message that "you will get what you came for"--required course material will be presented, practiced and consolidated and  revision, quizzes, tests will be periodic and will constitute an integral part of classroom rituals.

Over the period of time when you meet with your learners, some informality may be required as learners need to open up enough to give you pertinent information about their interests, lifestyle, goals and dreams.     Part of your work requires that you learn who the individuals in your classes are, especially when project work is assigned and you try to encourage learners to pick topics which interest them.

How much information you wish to divulge about yourself is a subjective matter.  However, propriety requires that 'learner-centered' teaching be your focus.  So, our recommendation is that you give your learners just enough information about yourself to provide them with a model and to encourage them to talk about themselves!

A word about the “sharing” on Facebook and  other social media:

If you choose to accept learners as their friends on your Facebook page, you might inadvertently share more information about your personal life than you would in other forms of communication.  Consider starting another Facebook page which you invite your learners to join.  There you can share in a friendly and open way, always mindful that you are their teacher and they, your learners.