As ministers, we all know too easily the arguments to not "friend" members of our congregation on Facebook and other social networking sites. It blurs the professional boundary we try so hard to establish. It leaves you open to people seeing something you don't want them to see if a friend tags you with an embarrassing photo or video or comment. And people will assume you know things they've posted there and forget to tell you. It's definitely a valid decision to not friend, particularly if the privacy controls overwhelm you. All of this is true.
But if you're comfortable playing with the security parameters, most of these concerns can be mitigated. So here's some of the other side from someone who does friend congregation members.
Pastoral Care: I don't see everything that members post, but occasionally I do see pastoral care needs on Facebook that I'm better equipped to respond to for having been a Facebook friend. For example, when a member was dying a while back, I routinely checked in on her family's posts just to see how they were doing, and it kept me in touch with the situation in an extra way.
Denominational Connections: In my little church, as with many, it's very rare for members to attend General Assembly. It's rare that they even attend District Assembly. They're not on the UUA's e-mail lists for different topics, either. The UU World is practically their only connection, other than me, to the larger UU Association. But a huge number of them are on Facebook. And so recently I went to the Standing on the Side of Love Facebook page, and clicked on "suggest to friends" and then looked at my church friends group. Only about 6-7 of them were already fans of the SSL page. So I sent all the rest a Page Suggestion. Now 45 members this group (some of which are members of the church, and some of which are friends) are followers of the SSL Facebook page. That's a population equivalent to half my church that's now connected to this important social justice arm of our association. Some time ago I did similarly with the Michigan UU Social Justice Network, and many members responded. It's an easy way to get your members so that they're seeing some of what is going on in our movement regionally and nationally. It's definitely not this easy to send out a page suggestion for a page that you don't run if you're not friends with the people you want to invite. It might be possible, but I think you'd have to do it one person at a time.
Communication: Once your members are your Facebook friends, it's easy to get pretty instant feedback on new ideas, by posting a discussion topic for them to comment on, or to communicate with groups quickly and easily. Some members are much easier to contact on Facebook than by e-mail. You can use Facebook's message system, or just sometimes post a note on their wall.
Evangelism: Facebook is a great soft-shoe evangelism tool. Your friends can be promoting things you're doing without even doing anything, because if they make comments it can come up in their friends' newsfeed, or your page or person can be suggested to their friends just because they've connected to you. This particularly is an important way your message can spread through your church's Facebook page, but it can also be done through your personal account. But imagine that you post an article that you wrote about something interesting to your personal account. A friend can then "share" that note, if you've left it unlocked for sharing, with their friends, who then will see the things that you're saying. If your note then connects back to the church in some way, then their friends have learned a little something about Unitarian Universalism and its beliefs. And nobody had to go knock on their neighbor's door and say, "Let me tell you about Unitarian Universalism." And maybe, just maybe, we can stop being the "best-kept secret in town."