I prefer twitter as a professional online network and have recently become more involved with the library world on twitter. Librarians sure do like to tweet! I joined twitter a few years ago and have primarily used it in a personal context. However, I did set up twitter accounts for Etsy shops I used to run.
I like that twitter allows for incorporating personal and professional styles. I still tweet about personal things, political things and somewhat trivial things like tv shows and tell silly jokes, but I also take part in and host twitter chats about LIS.
I help to run the blog, facebook page and twitter account for SLIP Ireland (Students, Librarians & Information Professionals Ireland), a group formed and run by postgraduate students studying library and information studies. Hosting twitter chats for SLIP has been a brilliant experience, you can have a look at past ones using #SLIPIreland.