This is the third in a series with those whom I've had some great conversations in workshops and meetings. The preamble in the first post of this series explains what I have in mind. I hope you'll check back regularly or, better yet, subscribe. You can do that either via the RSS feed or email in the right sidebar.
Related posts:
If you were at the meeting I had the pleasure of helping with a couple of weeks ago, then you'll recall I opened with sharing a picture of Brian Solis' and Jesse Thomas' The Conversation Prism.

I typically like starting with this one because I think it helps me to help others acknowledge, right from the start, the sense of overwhelm some folks feel when first getting started with all this social media stuff.
The key message: It's okay if you sense a little confusion about where to start. The fact is, you're not alone. And, it's no wonder anyway. I mean, geez, look at the landscape!
Hubs and Spokes
So, having said all the above, and while keeping in mind the diversity suggested in the pic above, I typically recommend, when starting out, that you don't try to do a "random walk" through the social sphere. Instead, if I have to pick a few to recommend starting off with, I'd say they'd be those shown in my "hubs and spokes" diagram below.
The list of platforms shown above probably isn't surprising for those of us who already have accounts with LinkedIn, Twitter, Facebook, YouTube and a blog (in the middle). They're the popular platforms today. (But with the rate at which things change, check with me next year and I might have a different list for you.)
When discussing these networks, the thing I point out isn't so much the platforms themselves, but rather their placement, conceptually speaking, relative to each other. More specifically, the placement of social networks (outposts along the spokes) relative to your blog at the center of the diagram (the hub).
Outposts (Spokes)
The general idea is that I typically use the metaphor of outposts when thinking about the networks along the outside of the diagram; those along the spokes. These represent culturally distinct places where you engage folks in places where they prefer to hang out. And I say culturally distinct because each network has its own set of shared attitudes, values and practices. (LinkedIn feels different than Twitter. Twitter feels different than Facebook, and so on.)
So that means that, in a sense, you kind of have to dress and speak for the occasion. You wouldn't want to show up to a business meeting, for example, dressed in garb you'd otherwise wear to a barbecue. Nor would you risk alienating new business contacts by sharing with them old high school pictures, vacation photos or "Mafia Wars."
Since I seem to be rife with metaphors today, let me offer these:
LinkedIn. Think of this as the iconic business meeting. LinkedIn caters to a user-base of business professionals. As such, the culture tends to be more formal with some users opting (correctly, in my opinion) to reserve statements, updates and information-sharing to those having a professional tone.
Facebook. This, to me, is like your backyard barbecue. Unlike LinkedIn, Facebook's network culture tends to be less formal. Now, it's important to remember that "less formal" doesn't mean stupidly whimsical. Don't post your beer-binging grad nite party pics here. Instead, when you think of "less formal," imagine a barbecue you host in your back yard where the invited guests are past clients, future customers or business colleagues. Everyone is free to wear shorts and drink beers, but keep the dialog at a business casual level while we each try to get to know each other better outside of business suits.
Twitter. I've heard the metaphor of a cocktail party having been used for Twitter. I think that's right-on. As with live/brick-and-mortar type cocktail parties, you'll likely find a lot of people milling about engaged in small-talk. Many of the attendees may, or may not, know each other personally. It's a place where you "walk around" engaging in light banter until you find a conversation that's more deeply engaging. When that happens, you could "hang out" with those folks a little more and maybe suggest follow-up meetings in a different venue where you can learn more about each others' interests. (Hint: Suggest venues such as LinkedIn and/or Facebook!)
YouTube. Ha ha. This one's a tough-y since it's that interesting mishmash of professional content juxtaposed with other topics ranging from informal to outright foolish. Yet, here again, we have to consider the culture. YouTube's culture makes what might otherwise be inappropriate in a business context to something perfectly correct in the larger YouTube environment. But, since my talks are usually geared towards the middle ground, I typically couch YouTube using the metaphor of a video shelf. It's a spot where I place my personal/professional video collection which I might occasionally choose to share with others who come over to my place for that barbecue (i.e., through Facebook) or business meeting (i.e., via LinkedIn). It could be that your collection includes topics ranging from personal commentaries on professionally-appropriate topics to business presentations or "how-to" tutorials.
The Hub
Your blog. From an online hubs-and-spokes context, I like to think of whatever is at the center of it all as that thing that best represents you. Think of this as a place online where you take the opportunity to reveal to the folks who click-through from one of your outposts, something about your personal and professional values, interests and passions. Any advertising or cross-promotional messages about your business services and offerings are best limited to sidebars where visitors can choose for themselves whether or not they want to participate. Ideally, they will do so after having sampled the topics on which you write in your blog. In a sense, they're indirectly assessing whether or not they share your values and interests -- whether there's sufficient trust to do business with you.
The hub should also be a site that is search-engine friendly, easily linkable and frequently updated. It's for all these reasons that I usually suggest a blog--or your web site with a frequently updated blog attached to it--as being a key feature at the hub of this framework.
If frequently updated with quality information, your blog will build inbound links from the simple fact that other people will find content they relate to and will want to share with others. As for content that helps you build trust with others, blogs are one of the best vehicles for indirectly communicating your personal and/or brand characteristics to your visitors in a way that doesn't blatantly shout "do business with me" to their face.