A few reasons. The first is historical. When the world shifted from portals to search, Google was the big winner. Now the shift is from search to social, with Facebook as the big winner. The mega-trend is Portals → Search → Social. That's the big defining shift on the web and we are at the very beginning of the transition to social.
Portals are about impressions, search is about queries, and social is about sharing. It turns out that sharing is a richer, more human currency. Portals have devolved into endless pageview-generating slideshows adorned with banner ads that don't work, videos with annoying pre-roll, and fluff stories that are interesting to everyone and no one. Search results are polluted with SEO gaming sites and pages that rank high that are more readable to a robot than a human. Social is avoiding these problems by directly measuring human actions. That is a better signal because humans are what ultimately matter. Facebook has a huge opportunity to build a defining company that directly impacts people's lives.
The second reason is that Facebook is epically successful at inspiring user engagement. According to Foursquare/Tumblr/Twitter/Zynga investor Fred Wilson, the best social media companies and services manage to get around 30% of their users to stay active each month and 10% each day. He says these numbers are basically a "law of physics."
That means Facebook is breaking the laws of physics. Facebook simply blows away every other social site when it comes to engagement. The majority of Facebook users stay active and their daily active user numbers are MORE than half their monthly numbers, meaning the MAJORITY of people login each day. It is freakin' crazy. And this engagement is happening at the scale of almost a BILLION active users, not just among social media folks or hipsters or celebs or any particular group. Facebook is used by more people, more regularly, with higher engagement than anything we have seen in the history of the web.
The third big reason is that Facebook solved a huge problem. In the 60s and 70s social scientists like Stanley Milgram hashed out concepts like “6 Degrees of Separation,” explaining how everyone is connected to everyone else on Earth through just 6 steps. This research has since been elaborated on by Duncan Watts and others. The concept of “Small Worlds”—networks where each node can be connected to any other with a few short hops—are at the core of the new “science of networks” that has emerged in the last few decades. More here and here and here.
In Milgram’s day, it was true that in theory each of us could reach anyone in the world through a few short steps. But in practice there wasn’t an easy way for information to spread from person to person. So these were intellectual curiosities without practical applications. But then the web came along, starting with email, and everyone could reach everyone else on the network.
One curiosity emerged, namely "email forwards," where a message created by one person could get forwarded from friend to friend and reach millions of people through sharing. For the first time, this highlighted how the 6 degrees concept could matter in practice. A new form of communication and distribution was created — social distribution that made viral media possible. It was thrilling but messy and broken! The same person would get an email forward 20 times, spam and fraud was rampant, Bill Gates wasn't really giving away his fortune, that Nigerian guy wasn't really a Prince in exile, and headers with FWD: FWD: FWD: FWD: FWD: weren't exactly elegant.
Part of the problem was the decentralized nature of email and the inability of anybody to see the structure of the entire network from a god's eye view. Facebook is the first company to fix the mess and deliver on the promise of these powerful ideas about networks. By combining true identity, the social graph, and newsfeed, Facebook had all the pieces it needed. Users can share things they love with their friends; News Feed can remove duplicates, filter based on your interests, and improve the experience based on data across the entire network. Stanley Milgram is probably doing back flips in his grave.