One of my fellow Office 365 MVP’s Martina Grom (@magrom) wrote an outstanding blog post comparing Yammer with Exchange Distribution Lists as well as how to utilize Yammer to replace Exchange DLs. I highly recommend this blog post as a great resource for moving into Enterprise Social with Yammer, http://blog.atwork.at/post/2014/12/07/Comparing-Yammer-to-Exchange-Distribution-lists.aspx