Describing what makes these sites popular is a pretty tall order, but I'll give it my best shot!

First, let's start with an overview of what these sites actually are, and what you can do with them.  While Facebook, MySpace, and similar sites all offer different features and appearances, their primary task is essentially the same -- to connect people to one another.  The sites are free, and all you have to do to get started is sign up using their free subscription form online.  Once you are signed up and logged in, you can begin to build your "profile" -- entering information about yourself, uploading photos, and more.  Other users whom you deem your "friends" are allowed to see your profile, write responses to your published photos, and more.  There's much more to these sites, but that's the basic idea.

So what makes these types of sites popular?  Well, it's easy to understand why they are popular with kids -- what is more important to kids than their friends, and self-expression?  These sites offer the chance to connect with friends AND express yourself, to the entire Internet.

But these sites are not just for kids.  I used to pass these sites off as being mainly just for kids, especially MySpace which allows you to modify your profile so much that it becomes nearly unintelligible.  And MySpace is still my own personal least-favorite, because it's just not as "clean" looking.  I most recently found time to create a profile on Facebook, and right away I felt more at home.  And, many of my own friends have ended up on Facebook, including most of my classmates from Norwich class of 1988.  It's certainly made it possible for me to get and stay in touch with what everyone's doing, practically on a daily basis -- people have "status" text that they can update regularly, that all their friends can see, and there's even an Instant Messenger application that allows you to chat with friends when they visit Facebook.  The photo uploading feature is great, and allows people to comment on your photos quickly and easily. 

I've found it to be a lot of fun myself, even though I haven't had much time to really "express myself" beyond a few photos and comments I've made to other people.  Some elements of Facebook I've found a little annoying -- they have additional "applications" you can add to your profile, and invite other people to add, and I'm just not interested in most of them.  Many are game-like in nature.  My advice is to start out on Facebook simply "ignoring" the invitiations you get to add certain applications, until you've seen how other members/friends have used them.  if there was a global "ignore all application invites", I'd probably turn it on!  I still use Facebook mainly for staying in good contact with my friends, family, and classmates, and I don't have time for lots of other games or profile customizations.  Anyway...

Many COMMERCIAL entities are also climbing on board, but the smart ones are doing it with great caution -- we are already advertised-to enough in this day and age, and nothing will turn off a community of users faster than advertising directly to them via their social networking site.  Rather, the smart companies are simply putting information out there for people to discover, and using the sites to put a "human face" on their commercial entity.  Many even field questions about products or services, right through their profile.  Combined with blogging, these companies have found a whole new way to get information out to the masses -- by going to where the masses are, online, and becoming a (discreet) part of the online communities.

As with any Internet site, you want to be careful what information you publish; it's probably not a good idea to list phone numbers, physical street addresses, or other private information.  But that doesn't mean you should be afraid of these sites, either... I encourage everyone to check them out -- create a profile, log in, and invite your friends to join, too.  It's quite an experience!

Going forward, I think these types of sites could become more popular with Baby Boomers, too.  Especially those sites that simplify their interfaces.  I think this is probably where sites like ESP are headed, in fact.  ESP has had some of these types of features all along, and now that it's free, it's even easier for people to join and start "expressing themselves" through biography information, poetry, and photo albums.  Adding features that will help members connect with their friends and family will be the natural progression.

Other people have discussed and written more extensively on this subject; here are some links you may find interesting:

There's many more links available, just Google "why is facebook so popular" (or MySpace, whatever) and check out the results.

Links to the popular social networking sites:

As always, comments are welcome!

Marc