Lately, I've been getting two similar appeals in my business e-mail: from LinkedIn members wanting to add me as a connection, and from Facebook members adding me as a friend. So now I'm wondering: Which network should I actually take the time to cultivate?The two networks are actually not all that similar -- although they're trying to be. LinkedIn has been around for a few years, and has, from the beginning, concentrated on helping professionals make connections in order to further their careers and find (and keep) jobs. And in some cases, it works: I picked up a very good freelance gig some time ago when a colleague unexpectedly got back in touch via LinkedIn. (A colleague who probably wouldn't have thought to e-mail me directly, despite David Pogue's bemusement.)

Facebook, on the other hand, started off as a place for students, and has only recently been encouraging business people to create their own Facebook networks of friends and colleagues. However, because of this background, it is also a much richer and more varied service than LinkedIn (which has been working lately to remedy that by adding several new features). Facebook users can add a steadily increasing number of third-party applets to their Facebook pages -- I've only just begun to investigate what's there, but I've been found that they range from the useful (such as Causes, which makes it easier to find charitable and political organizations to join and donate to), to the interesting (like the Interactive Friends Graph, which offers an image-based method for graphing out your friends list), to the outright silly (Harry Potter Magic Spells -- share it with your kids).

While I don't want to count LinkedIn completely out yet, I can't help comparing this transition to what happened when computers moved from 5.25-inch floppy drives to 3.5-inch floppy drives. (For those of you under 25, floppy drives are what we older folks used before USB flash drives showed up.) For a while, computers were manufactured with both types of drives as users coped with moving their files from the older to the newer format. Eventually, though, the larger 5.25-inch drives disappeared.

Of course, the world of Web-based services is a lot more volatile than that of computer hardware. While Facebook is currently the flavor of the month (having replaced Twitter on top of the "you-gotta-try-this" menu), it could be replaced tomorrow with some up-and-coming startup. Or LinkedUp's team could come up with some new innovation that will pull back the wandering attention of the business community.

In the meantime, I'm resigned to fielding requests from two (or more) social networking Web services. But if it means that I may find several friends whom I'd otherwise lost track of, then it's worth it.