Your customers know which mediums they like to be contacted through – and they’ll be happy to tell you.
Some customers prefer email. Others like social media. Some prefer the personal touch of a phone call. Others would hate to be called, but are happy to receive material through the mail.
These preferences can vary a lot according to the demographics of your customer base and the nature of your business. However, in all cases the best way to find out is simply to ask your customers how – and how often – they would like to be contacted by you. Make sure you listen to their answer, and respect their decision – let their contact with you guide you on this.