Unlike, say, the email that keeps piling up in my Inbox, available time for social media is not an ever-expanding commodity. Which means that even though engaging with readers on social networks is a key part of my job, it's getting close to triage time. If social media isn't a core part of your job, you may be even closer to social media overload and looking to decide where best to put your efforts. Here's what I find to be the strengths and weaknesses of each:
We're having nice success with our Computerworld page, but I can't say my Sharon Machlis author page is doing much for my own personal brand (if you want to help change that, please do give it a "like" ... ). Perhaps I'm not using the platform to its best advantage or spending enough time and effort there -- I'm not a big fan of the overall Facebook environment -- but my sense is that Facebook is best either for personal connections or corporate brands and less so for connecting with individuals (who aren't celebrities) you don't know personally.
Critical mass. Many people joined Facebook simply because so many other people and businesses they know were there. It's hard to argue with 750 million users. And it's not just numbers -- people tend to use the site frequently and stay for awhile.
Keeping up with family, friends and acquaintances.
Brand promotion. When users "like" your company's Facebook page, they are in essence advertising it to all their other "friends."
Internal analytics. It's fairly easy to get stats about user activity on a corporate page.
Ease of engagement. It's exceptionally simple for users to express an opinion about something by clicking a button. Additional comment is optional, but that one simple click advertises the item to a user's social circle.
Segmenting your life. Google+ appears to be focusing on this issue by organizing around "circles," allowing an easy way to target particular posts to friends, family, colleagues, etc. In fact you can do this on Facebook as well, but Facebook lists are not as easy to find or edit; nor are they as easy to view as a separate stream.
Privacy. With changing privacy mechanisms and unannounced changes that occasionally undo prior protections, it's best to assume that anything and everything you post on Facebook is public. That's fine for most professional and personal branding activities, but less so for a lot of other types of personal and professional communications.
Data export. If you ever want to leave Facebook, there's no way to export your content short of manually scraping page by page. The only way around this is to keep copies of everything important you post on Facebook in a single location, so you can re-create content elsewhere if necessary. Correction: As several commenters pointed out, Facebook has in fact implemented a way to download information from your account, by going to account settings.
Following issues. If there's an elegant and useful way to follow topics on Facebook, I've yet to find it. Yes, there are interests and pages you can click on, join, like and/or follow, but I haven't find many to be especially useful.
Environment. With an old-style interface, personalized-occasionally-to-the-point-of-creepy advertising and endless game app posts that need to be quashed, it's no wonder that Facebook has a fairly low satisfaction level despite its high level of usage. That doesn't necessarily mean Facebook is ripe to become another MySpace, but it's something that bears watching as the Google+ vs. Facebook battle plays out.
Following interests. This is still my preferred platform for finding and following interesting people and issues. I'm much more likely to follow someone I don't personally know in my Twitter stream, and I also regularly look at hashtags like #dataviz and #gis to keep up with topics I like.
Twitter is also excellent for creating temporary communities around events by use of hashtags.
Brevity. Twitter's short-form restrictions is ideally suited for an era of microscopic attention spans, allowing people to quickly scan for items of interest and click through for more when they want to.
Brevity. The 140-character limit that's appealing for some is a frustration for others. Not every important point can be whittled down to a tweet.
Potential overload. If you've got a robust list of people you follow, you'll likely miss a lot. There's so much content streaming by, it's simply impossible to catch it all. If you're a business using Twitter, you usually can't count on more than a small fraction of your followers seeing any given tweet.
Platform itself. Twitter has made great strides in building out its infrastructure to handle traffic loads and reducing appearances of the "fail whale," but when it comes to user experience, innovation seems to be happening more among those building clients or services around the Twitter API than within Twitter itself. Yet relying on a small, third party service can be risky, as I discovered when Twitter purchased Backtweet and shuttered its API -- so much for my internal tracking tool that tallied which of our stories was being tweeted most frequently.
Analytics. While there are plenty of ways to measure the effect of your own tweets, measuring how much traffic Twitter is generating overall can be difficult, since so much of it comes from desktop and mobile clients and not via twitter.com.