Contacting your customers/members

Employee at a credit_union ($291MUSA)
We are looking at different avenues for contacting members/customers regarding transactional info on their account; not marketing.  We have some that are not responding to mailed letters or phone calls regarding their dormant accounts and we are considering emails.  We have read the CAN-SPAM reg and will not be marketing to them and will still include the option to "opt out".  We have staff that want to use facebook messenger, but I don't feel social media messaging is the way to go.  Do you use any social media messaging to contact your members?  Do you have a sample of your authorization that your customer/members sign?  We will be running this by legal, but wanted to see what others are doing as we update our policies and procedures.