Facebook claims to have more than 400 million active users. In fact, according to Web analytics firm Alexa, only Google is a more popular site. So, with all that going for it, why are so many users unhappy, with one poll showing that more than half of Facebook users are thinking about leaving?

The one-word answer: privacy. By default, Facebook has made almost everything on your account open to the world. You can lock down your Facebook account, but it's a tedious process.

New tools unveiled this week are designed to make this easier, but the next time Facebook changes its policies and system, you may need to do it all over again. There are also external tools that can check out just how secure your account is, but again, when Facebook changes its software, they may not work.

Last Monday, Facebook CEO Mark Zuckerberg finally admitted that Facebook has made some mistakes. Some users are staying put despite their distrust of the company. But others think it's too little too late and are talking loudly about leaving. There's even a group that has announced that May 31 is Quit Facebook Day.

Will a significant percentage of Facebook users actually leave? No matter its faults, Facebook has a huge user base, and those users, in turn, have all their friends on the service, all their photos and, yes, even all their Farmville farms. Will that many people really want to abandon Facebook and start all over with a new platform?

They have before. In social networking's early days -- all of five years ago -- Friendster, MySpace and Xanga all had their days in the sun. Now those once-popular social networks are in decline.

There are, of course, the other existing social networks. But some, like MySpace and LiveJournal, seem to have missed their moment. Others, such as the professional-oriented LinkedIn, have established a specific niche or, like Twitter, address different needs.

The alternatives

A group of would-be Facebook replacements have recently raised their hands, hoping that enough disenchanted users will see them as viable alternatives. Some are already out there, some are in beta, and some have hardly gotten past the "what-if" stage.

Who are these players, and do any of them have what it takes to become the next big social network?

While this open-source project is still in beta, it's an interesting take on social networking. Instead of being under the control of one company and one set of administrators, Appleseed works via a distributed server software package tied together with the ASN (Appleseed Social Network).

That means that as a user, you select and log into an Appleseed site. Once there, you connect with friends, send messages, share photos and videos, join discussions and participate in all the usual social networking stuff. Don't like the specific Appleseed site you're on? Then sign up for a different one and, according to the site, "immediately reconnect with everyone in your network."

It sounds good, but I can't help but notice that even though the program is officially in beta, I couldn't find any ASN sites to log into yet.

In a recent New York Times article, Michael Chisari, an Appleseed developer, said that the project is six months away from opening its doors to the public. It looks promising, but I'm not holding my breath on seeing Appleseed give Facebook serious competition anytime soon.

Diaspora, for all the headlines it's gotten, is still not much more than an idea. Mind you, with almost $200,000 of support from thousands of donors at Kickstarter.com, it's also a very popular idea.

Technically, Diaspora sounds a lot like Appleseed. It's also going to be built from open-source software, and it's going to be a distributed network server application. More power to them, but at least Appleseed already has some code.

The first Diaspora code release is slated for September.