Sanctify yourself though that which is permitted to you. (Babylonian Talmud, Yevamot 20a)
A blog (short for "web-log") is an interactive online forum. It's easy to create a blog, free, and doesn't require much skill beyond word processing and email. The main reason for a synagogue to create a blog is to offer, online, the equivalent of a meeting place or community court. We all strive to create opportunities for our congregants to connect with each other in fulfilling and meaningful ways. The blog, when thoughtfully used and maintained, can offer a chance for this to happen.
A blog can also enhance what's already going on in the synagogue. Is a great discussion happening on a regular basis in a Lunch and Learn or Torah Study class? The blog offers a chance to expand that discussion to anyone and everyone, wherever and whenever they want.
There are many "administrative" uses of a blog as well, offering many easily updatable options for sharing information in a one-way manner updating Religious School calendars, reminding about upcoming events, sharing sermons, calling for volunteers, etc. These are useful but are merely an extension of the congregation's website. A truly Web 2.0, interactive, blog offers a chance to build community in new and different ways.
Measuring "return on investment" for a blog or other social media is difficult, if not impossible. Don't expect your blog to bring in specific, numeric quantities. The goal of a blog or other interactive media (like Twitter, Facebook, etc) is human interaction and conversation. These are not quantifiable entities but instead contribute to the overall community-building mission of a congregation. If a blog is merely a list of events or a "one-way street" information going out but not coming in, it's unlikely to be fulfilling its purpose of fostering two-way communication. Embarking on a plan for using social media in your synagogue is not about numbers. It's entirely about creating and joining in conversations that enhance the community that already exists. See the blog as one more tool in the kit that helps us to connect ourselves to our congregants and our congregants to the synagogue and the greater Jewish world.