Not being online!
I am not saying that you have to be some power user always posting and tweeting, nor do you need to allow social media to take over you life. But I am suggesting that you should have a active, consistent online presence. Pastors can't afford to be absent from the "Areopagus" of our culture (Acts 17).
Social media channels such as Facebook, Twitter, Linkedin provide incredible opportunities to:
Connect with those in your church - these social media channels are the places where conversation and sharing among those within your church are already taking place. Therefore a place, such as Facebook, can provide an opportunity to know and discover the spiritual/emotional pulse of those to whom you minister to. Also, you can encourage and inspire those you shepherd by posting inspirational quotes, articles and verses through your social media.
Learn from other pastors and thought leaders - being online allows you access to an incredible learning community. Being online allows you opportunities to read, to be inspired and to learn from other pastors and ministry leaders from around the globe - and it's easy to do. For instance, you can read from other ministry leaders' blogs using RSS readers such as Feedly and you can follow and read from various pastors and leaders on Twitter.
Manage your professional reputation - being online allow you to define and manage who you are, what you believe, and what you do. There is a lot of trash and misinformation online. Therefore rather than simply allowing other's to talk about you online, being online allows you to be proactive and allows you to define yourself.