'CONSISTENCY' is key to social media and email marketing. These are powerful tools but you have to be CONSISTENT with them. The secret is - do it often enough so has not to lose contact with your customers, and send them 'content' that is valuable enough to make them want to stick around and not 'opt out'.
So what is that magic formula? Is it once a week, twice a month, everyday? It varies and depends - on time, the message, the business, the customers and maybe even which Social Media/Email marketing tools are being used.
The important thing is CONSISTENCY. Out of sight, out of mind, as the saying goes. So be CONSISTENT.