I agree with all of the above, but especially with @dj_justjay about running the risk of being inefficient. You have to have an agreement and understanding about how communication will be executed between the agency and company. When someone makes a comment or asks a question, it needs to be answered very quickly - within hours at the longest. If the agency has "the keys" 100% of the time and isn't able to communicate with the client quickly, it can't work. I helped one client who often left her cell phone at home and didn't check her email every day. Needless to say, there were lots of communication problems. These details have to be discussed before the plan launches.

I think that you have a question to ask yourself, Susan: Is social media right for my business? I'm not saying it's not, but before you jump in, it's important to first have a clear idea of how your target uses social media. If I had a business that targeted seniors, I might get some insight from my existing senior customers on how it would work for them. Do they even use it? This will allow you to develop a clear plan on how to engage. Maybe you would discover that it would be a complete waste of your time, or that it could be effectively used in a way you hadn't thought of. Hope that helps.
My recent post Social Media Revolution