Your policy will have to inform users about what data you are using and why. Depending on the type of data you are collecting and using, you may also be subject to more stringent requirements.
The policy also needs to be accessible to users. You will have to provide users with some type of link where they can read the policy and update them with any changes that you make to the policy going forward.
Terms and Conditions, on the other hand, may not be required but it would be a good idea to go ahead and add those too. The Terms and Conditions will govern how your app is used and are very important to cover yourself from potential wrongdoing by users. In short, going without Terms and Conditions, especially in a social app, could subject you to liability for the actions of your users.