I’d like to also propose that the intended audience of these sites be considered. Generally we could say that the demographics of a Facebook & Twitter user are much younger than those of most (not all mind you) LinkedIn users.

If you were to compare just 2 of these sites ; say Facebook and LinkedIn you could easily see the target demographic that each site was going for just based on there INITIAL designs. For example:

1)Facebook – honestly took a page out of Napster & AIM books when it came to the “sharing concept” , but instead of music this time it was everything you could possibly imagine. Facebook allowed you to build your own system of people you knew friends, acquaintances, co-workers and share anything you wished ; photos, feelings, warnings, you name it- all almost instantaneously; thanks to a constant RSS feed. You could alert anyone you knew of anything at any time. And if there wasn’t an application for what you needed ; no worries you could build it yourself and post it to show everyone. This feature alone ensured the continual growth of Facebook ; for it made the user a part of the growing/ learning process.
A.This design (or corporate strategy) endeared itself to the tech savvy, innovative, to adventurers and travelers. On the whole, when you looked at this group the demographic was overwhelmingly young ; and all could be considered to have the early adopter mentality. But there loyalty was unquestioned. They went back to their homes over breaks and summers and introduced siblings and parents to the site. And thanks to the feedback of those users Facebook grew and still maintained it’s usual ease of use (thought not it’s original layout.)

2)Linked In’s initial platform mimicked many commonly used job search engines (similar to Monster.com and Careerbuilder.com) Using these as a reference for their user interface they instantly gave the use are sense of ease when navigating their system, as they most likely all ready had experiences similar to this. They then took the social networking success from myspace and facebook (which both debuted on college campuses) and incorporated it into their site in many of the ways listed above : internal job postings, visible professional references on your profile, etc…
A.This design (basically that of a job search engine) limited it’s audience from the get go. By limiting it to strictly professional networking they instantly cut out the college aged demographic- or to look at it another way the workforce of the future/ early facebook adopters. Specifically they lost a huge opportunity for work of mouth advertising .
B.While LinkedIn has changed overtime and now encourages professional forums and college alumni associations initially it didn’t have a place for user feedback; which is invaluable in todays world. ( What CEO would say no to FREE customer feedback?)

Now given these ideas it’s easy to see why you aren’t liking LinkedIn. Simply put Kel , even though I have never met you , everyone knows you’re an early adopter . Whether it be to technology, advertising, public relations or marketing. Therefore it is not surprising that you wouldn’t like LinkedIn (which may come off as the electronic equivalent of an old boys club sometimes.) There is no challenge to it, limited change, and no real driving content. Nothing there to stimulate of satisfy.

Therefore let me entreat you to come back and play with us “kiddies” on Facebook. It’s much more fun; plus we have cookies!

You say, “I am happy to make an introduction for someone I respect and/or care about, but I would never leverage an impersonal third party, like LinkedIn, to do so.” I’m happy to introduce friends as well, but I don’t always realize they’re attempting to connect with someone I know.

I’ve recommended LinkedIn’s “Jobs insider” application to my job searching friends and use it myself. With the skyrocketing amount of applications posted jobs are receiving, it’s more important than ever to find a way to stand out. If LinkedIn shows me that cousin Susie went is connected to Jane Doe hiring manager at a dream company, I’ve found a connection that I may not have realized existed without LinkedIn’s help. I can then e-mail Susie to find out how she knows the contact and ask for an introduction.

It also feels good to be able to connect job searching friends with contacts that may be able to aid them with their searches.

Facebook has triple the users, an open platform and a position in the culture. (Business model? Why would they need that?) LinkedIn is a niche player, but as Laura points out, it’s a niche that some people find useful. In defense of LI, I have found some good discussion on their forums, and I have hired good peeps there. Of course, the same can be said of Craigslist.

I think smart social media peeps will take the Let a Thousand Flowers Bloom approach and use syndication to pipe their content into every dark corner of the net. Try using Ping.FM to push your FB status or Tweets into LinkedIn and the LinkedIn blog applet to add your blog entries to your profile. That way those who insist on living only on LinkedIn can enjoy your brilliance too.