Today Facebook went public, applying for its initial public offering, hoping to raise $5 billion. In 2011 it made $1 billion of profit on $3.7 billion of revenue. With 800 million users it's a formidable force on the internet. So we think.
So why do I think Facebook will be gone within the next 5-10 years? I have a couple reasons.

First, Facebook has become a massive directory of people. It's not a personal social network anymore, rather it's a giant address book of sorts where instead of having your favorite people in your contact list like on your cell phone, it's like having the entire phone book on your phone and you have to sift through people. Each of them have a diminishing marginal value to you because there are just simply too many of them. While there are options to filter your friends, it's still a cumbersome and ugly process that presents little functionality. In a sense our life is on the web for ALL our 1000 friends to see.
Second, Facebook is not private. This goes along with the first problem, but is something I think is a fundamental failure of Facebook. I have read some economic papers that suggest many Americans abandoned the crowded cities of the 20th century to venture into the suburbs for the reason of privacy. Of course there were other reasons, but it's necessary to note that Americans love their privacy. Facebook does not promote privacy at all. Im fact with the recent timeline feature, all of our lives from the very moment we joined up on Facebook are easily shown, from those embarrassing photos to those comments where we use the swear word. All of which is quite embarrassing and presents us a chance to have our peers, and employers included, change their opinions of us. Not a popular thing there.
Third, going along further with the privacy issues, Facebook also is getting old from its original users and as we get older, do we want the same content from our distant pasts to GE broadcasted to the rest of the world? I don't mind because I don't have embarrassing photos or other content, but still, many others do. Will Facebook be the adolescent cool thing and as we get older and more mature, will it just be uncool? I think for many, it will.
Fourth, Facebook is a tool for the U.S. government to spy on Americans and other people from around the world. Given its big name, as people want privacy from this increasingly invaded private lives, they will venture to smaller networks that are less watched and considered underground so to speak.
Finally, I just think people have this trend to smaller private networks that will make each friend more valuable than just a number on your friend list.
Google+ is what I think a future model, but will have to have other services in order to work.
I like Facebook to an extent, but I would like an alternative too. I think others will agree and over time, Facebook will be just another giant to meet its sunset like other former Internet giants.