When it began, Facebook only had some very basic features - you could post photos, include profile details and contact information. In the college social environment in which it gestated, Facebook created many connections between users very rapidly. Undergraduates generally meet huge numbers of people in a very short period of time, so a tool like Facebook was something that fulfilled a substantial and unmet demand among collegiates.
As the application has developed and spread among people outside the university environment, it has become more of an important tool for professional networking. In the earlier days of Facebook, it was unfamiliar to most employers. As more of the people who went through college using it have entered the workplace, it has become an informal way for co-workers to connect with each other and for people looking for work to remain connected with other professionals in the same field.
It has also formalized what was once a quiet back-door network of alumni - many savvy Facebook users find good jobs through connections with people who they once went to school with.
It's also created something of a conundrum for people concerned about their privacy. Many people have been denied jobs or even fired because of the content of their Facebook profiles - most often people who work with children who have photo albums that display them posing for inebriated party pictures.