When once-mighty MySpace's fall from grace culminated in their sale for $35 million, a gaping hole was left in the social networking world.

The hole had been there for a while, but MySpace lingered and nobody was able to push them aside.

Facebook is the current and future king of the industry with nearly 800 million users and large amount of cash behind them.

If rumors of a 9-digit IPO come true, the site will maintain its spot for the foreseeable future.

Twitter is going strong. They are revamping their revenue streams regularly and continue to make news, particularly when there's other news being made in the world. The speed and news aggregation aspect of Twitter will help them to continue relevance.

Google+ is still an infant, barely out of the womb. Despite amazing initial growth, the newest major social network is still a blip on the radar, far below a dozen other social sites in total users. As more than just a social network, they are definitely a player in 2012 and beyond.

For the three established (or in the case of Google+, well-backed) social networks, the future is bright. For everyone else, there are questions that must be answered. MySpace is gone. There's very little that Justin Timberlake can do to right the ship. Of the second-tier social networks, which one can step up and fill the hole? It can and should be filled; as fast as the four big social networks are growing, they still are not growing fast enough to meet demand. People want more than the current options.

Who will they choose?

Those Who Don't Count

LinkedIn has been successful at very few things, but the one thing they have done well, being the preeminent social network for the business world, has made them very powerful in the niche. Still, it doesn't matter how they focus people on running status updates or posting links, it's always going to be the place to make business connections. That's it.

YouTube, Tumblr, Flickr, and other content sites can't count, either. People go to those sites whether they are users or not and they don't really fit in as true social networks.

Sites that have no real presence in English-speaking society do not count. They may be big in China or Brazil, but they cannot expand worldwide the way that English sites can.

With those out of the way, let's see who's left.

The Contenders

According to Hitwise, MySpace dropped this month to #6 on the largest social networks list. Facebook, YouTube, Twitter, and LinkedIn continue to take more of the share. Yahoo! Answers is not a social networks and probably shouldn't be on the list.

The four that are left plus two that are not on this list are all viable contenders.

Tagged.com, a social network that focuses on letting users make new connections rather than engaging with friends and family like Facebook and Google+, jumped ahead of MySpace nearly a month ago. The site has over 100 million users.

iVillage is on the list and does have some social features, but it's more of a large niche community site rich in staff-generated content rather than being a place to socialize. That may be exactly the difference that's needed to help them fill the MySpace spot.

My Yearbook seems to be a Tagged Jr. Same focus with fewer people.

My Life is a combination of many aspects of other social networks. It has a compelling hook on the home page and draws people in to sign up easily. However, once beyond step 1, it takes you immediately to an upgrade page. While this might be good from generating quick cash now, it's not a revenue model that is geared for growth.

Bebo is another site with over 100 million users. However, like MySpace, it was sold for much less than its original purchase price and shows signs of decline rather than growth.

Classmates and its 50 million users has little chance of dominating its niche when Facebook is already filling it.

When looking at these sites being within striking distance of the upper echelon, any one of them can make a change or cut a deal that could propel them. The chances are slimmer for some than others, but it will take an existing social site like these or a brand new one born from some other entity (if Mashable were to leave blogging and become a social network, for example) to take over where MySpace has left off.

Why There Needs To WILL Be a Fourth

Google+ is taking on Facebook in its own arena. It looks like Facebook. It acts like Facebook, albeit with a more modern interface. It has the same basic goals with slightly different functionality as Facebook.

Twitter is in a world of its own and will stay there for a long time. It's a venue for people to tell the news and for journalists to pick up on the news. It's a central point of scandal for many. As long as they do not squander the money they have behind them and focus on building a strong revenue model that won't drive away users, Twitter can remain untouched by other networks with no chance of anyone taking away their crown.

Over the next year, a portion of the growing social networking population will want something else. They will get "Facebook fatigue" over the same people saying slightly different variations of the same things. They will find something about Google+ that they don't like (or something about the company itself that drives them away). They won't find solace in Twitter.

This portion of the social networking world will want a 4th option. They wanted it before Google+ was born, before MySpace fell from grace. There simply weren't the right alternative for them to act, so they stuck (in many cases, reluctantly) to the mess that was before them at Facebook.

Verizon and AT&T dominate the cellular service market, but Sprint and T-Mobile are still necessary regardless of whether or not the AT&T/T-Mobile acquisition goes through.

The same holds true for social networks. There needs to be an alternative. The three major players can handle 95% of the social networking done in western society, but somebody has to step up and cover the remainder. 5% might not seem like a lot, but with the continued rise in social network adoption and activity, 5% is plenty.

Who will it be?