Whether or not your bank gets on Facebook, you should already have language in your employee policy regarding their use of social media.
Before you launch the bank's site, do a risk assessment and develop procedures to control the site. Think through the reputation and compliance risks. Who in the bank can post? What will be posted? Do the persons in the bank responsible for the site fully understand how to control the site, e.g privacy settings, restricting outside posts, etc.
You have to control this so that you don't get a lender posting, "Our 30 year rate is just 4%!" or Joe Schmoe posting a tirade about his ex-wife who happens to be a teller. Be sure someone monitors the site continually.
A local ice cream shop learn about facebook and reputation risk the hard way. The owner offended some high school kids in the shop. They started a facebook campaign against the store, posting on the stores FB page, and the owner has said business dropped substantially.
While one topic of social media, has everyone searched Yelp.com for your bank? It's a rapidly growing site with reviews of all types of businesses. Someone posted a glowing review of one of our competitors, including photos. But many review can be viciously negative.
I thought getting old would take longer.