Prior to about 2005, if you had something to say online, you built your own website and said it there. And so the web was like a chain of small islands, each led by their own leader (the owner of the site), with browsers hopping from island to island. Sure, there were travel agents (search engines) to help you find which island (website) you wanted to visit, but for the most part, each site was run independently and had its own way of doing things.
You could have your own website through a variety of sources -- many ISPs and most colleges and universities offered web accounts to their members/students. Additionally, in 1995, Geocities was founded, providing free web hosting to anyone who wanted it. Geocities even offered a WYSIWYG (What You See Is What You Get) editor for websites, so this allowed almost anyone to post their own website, even in the late 90s.
In 2004, Mark Zuckerberg launched "The Facebook" (now known simply as Facebook). At first, it wasn't a big deal -- it was limited to college students at a handful of universities. As the number of universites grew, so did the demand for accounts for the general public, and so, on September 26, 2006, Facebook opened its doors to anyone with a valid email address and the ability to pick a birth date more than 13 years prior to their registration. People ran to Facebook because it was the "cool thing to do," and by 2009, Facebook was cash-positive, and today there are some 800 million users of Facebook, which means that about 12.5% of the world population is on Facebook. This is mostly interesting because there are about 2 billion internet users worldwide, meaning Facebook has a market penetration of about 40%.
So, in 2006, this new service called Twitter came on the scene. Twitter was interesting mostly because it was the first "microblogging" service to really gain traction. Twitter posts are limited to 140 characters in length, which makes the success of microblogging rather interesting -- who would have thought that anyone could say anything interesting in 140 characters? (Of course, many would argue that most of what is posted on Twitter is not worth the 140 bytes required to store it, but that's another matter.) And, much like Facebook, as Twitter gained momentum, its numbers rose steeply. Today, there are 200 million users on Twitter posting 200 million "tweets" per day.
So we've seen that users have been running to these services like an oasis in the desert, and similar things are happening to email, at least for personal email accounts. First everyone had a Yahoo! or Hotmail email account, but when Google announced email with 1GB of storage, many people moved to Gmail. Even today, most of the personal email addresses are associated with one of those 3 services.
It's not just the users who are centralizing, either. Service providers are looking towards these free tools too, without a thought as to the motivations behind the companies offering the tools. Take, for example, Google Analytics. 49.5% of the top million websites use Google Analytics. How about Disqus, where 13 million users post in over 500,000 communities?
Obviously, these service providers require money to maintain their services, let alone to turn a profit. Facebook, Twitter, Gmail, Google Analytics, and Disqus surely aren't providing services out of the goodness of their heart or some sense of corporate altruism. They're not charging their users (except for some premium services). So what's in it for them?
Data.
Google gets a peak at the users of 50% of the world's busiest websites and a healthy percentage of e-mail traffic as either a sender or recipient. Facebook and Twitter get a snapshot into the lives of hundreds of millions of individuals, seeing a cross-section of human behavior on a scale never before imagined. Disqus gets to see what people are talking about and where they're talking about it.
Who might be interested in this data? Well, first and foremost, I suspect marketers would like to know a lot about this. Who's discussing fashion, technology, or business? Maybe executives would like to know how many people are talking about their company -- or their competitors. And just perhaps, less-than-friendly governments (left to the interpretation of the reader) would like a peak at what people are talking about, or even the easy ability to shut down the now-recentralized lines of communications among the people in their country. Look at efforts by Egypt and Syria to shut down access to these tools during their revolutions.
I'm not saying these services are bad. I use many of them myself. I have a presence on Facebook, on Twitter, a GMail account, and I use Google Analytics to help me better understand my audience. I do it because I make a conscious decision about what data I place where, and how I handle my personal information. I choose the benefits of the service against the privacy tradeoffs. We all make compromises, every day. I just want you to stop and think for a few seconds before you sign up for the next big service: "What am I giving them? What do they get out of this? What do I get out of this?"