This past week, social networking site Facebook trumpeted the fact that is signed up its 500 millionth member. That’s an impressive statistic — and all the more so when you realize that Facebook had only about 100 million registrants just two short years ago.

And the site is truly international these days, with ~70% of Facebook users living someplace other than the USA.

But there are some interesting rumblings in cyberspace these days that suggest the bloom may be off the rose for Facebook. After having climbed to the #1 perch in terms of registrations and site traffic, there are some intriguing new signs that all is not well in Farmville – or elsewhere in the land of Facebook.

Inside Facebook, an independent research entity that tracks the Facebook platform for developers and marketers, is reporting new Facebook registrations dropped in June to ~250,000. That may still seem like a lot of people, but it’s a far cry from the ~7.7 million new registrants in May.

Furthermore, looking at age demographics, Inside Facebook has concluded that in the critical 26-34 age group, the total number of U.S. users active on Facebook actually declined during the month of June.

Are these people being swayed by the privacy debate that’s happening concerning how much visibility Facebook postings are being given on Google and other search engines?

That may be one explanation for the decline, but there could be other forces at work as well. The latest American Customer Satisfaction Index report from ForeSee Results, a web research and consulting firm, places Facebook’s ranking near dead-last on a list of 30 major online web sites in terms of customer satisfaction with site design and utility.

Who scored highest? Dowdy old Wikipedia. Even boring government sites like the IRS scored better.

It’s evident the issue goes far beyond privacy concerns. There’s also confusion or irritation with Facebook’s ever-changing user interface. As Aaron Shapiro wrote recently in Media Post’s Online Media Daily:

“The truth is, Facebook isn’t fun to use anymore. It’s become a chore, just one more place that busy people have to log in to stay up-to-date. And Facebook is making the goal of staying up-to-date harder and harder to achieve. There are so many apps like Farmville producing status updates, as well as people using Facebook as their repository for passing thoughts and private/public conversations, I have to sort through tons of what I don’t want to read before I get to something I want or need to know.”

Back in its early days, the beauty of Facebook was that it provided such an easy framework to stay connected with family and friends. It was a way to share photos and other personal information quickly – and almost effortlessly – with far-flung contacts all over the world.

Those attributes seem to have gotten buried in all of the “spammy” hi-jinks and gimmicks that characterize so much of today’s Facebook.

Considering the growing dissatisfaction with Facebook, ranging from things like privacy (mis)management and ubiquitous advertising to confusion with the site’s ever-changing design and irritating lack of utility, some industry watchers are predicting that users will begin seriously looking at alternatives. Despite Facebook’s huge presence and large pool of registrants, they may find simpler, purer sites out there that are more to their liking. Several that could be beneficiaries of the “Facebook fall-off” are Diaspora and Collegiate Nation.