There are couple of apps similar to Instagram, such as Hipster, Pixl-o-matic, Lightbox, PicPlz & Retro Camera.
Why did Instagram make it over the others?
In many cases companies are acquired for their staff not necessarily their technology. Google and Microsoft often buy companies because they have a couple staff members that are brilliant and really understand their niche and business inside and out. The knowledge and benefits of having them part of their team over the next 3 - 5 years is often worth much more then the purchase price they are buying the company for today. Typically those buyouts have clauses for how the money and stock vests that encourage many of the employees to stick around for awhile.
Facebook is the largest online photo repository. Instagram accounts for a good portion of photos submitted to Facebook and Twitter I imagine. Twitter doesn't have a good photo platform, each mobile app pretty much uses it's own - so it could have been a defense purchase by Facebook.
But versus the other apps - Instagram was definitely more popular, it was more social in nature, and their team probably understands photo and media sharing better than most Facebook staffers. They might want some fresh blood and fresh perspective or it all.
Relationships, inside connections. It's all about who knows who, not about functionality or apps.
Relationships, as mentioned earlier, but also its large user base. And don't forget, it is more hipster.
Here is an interesting post on the subject. The author thinks, like I do, that was luck. A complete and utter fluke.
You should read this article "here-is-why-did-facebook-bought-instagram". In short, Instagram has what Facebook want.
http://gigaom.com/2012/04/09/here-is-why-did-facebook-bought-instagram/
Companies make acquisitions for all kinds of reasons - talent, customers, brand, technology.
The huge valuation number in this case makes me suspect that facebook simply wanted the category leader in the eyes of users, and that was Instagram. There was competition, and so Facebook used its monster cash pile.
I doubt there's a real business case for the acquisition pure and simple, but they've sent out the clearest possible message to startups that Facebook is in the market for popular and successful capabilities that extend their offering, and it wants to be a well-liked buyer.
Network...Network...Network. You need to know people who are good in tech, business development, ops, marketing, sales - anyone related to running your company
More insight in this deal by NYT: http://www.nytimes.com/2012/04/14/technology/instagram-founders-were-helped-by-bay-area-connections.html?_r=1&pagewanted=all