Facebook is the world's largest social network with around one billion accounts. Because the users of Facebook leaves out some information about themselves the advertising via Facebook will be easier to target to the right audience. You can choose according to age, gender, interests and more.
Facebook also offer different types of campaigns for different purposes and different platforms. If you need to advertise a mobile app it´s obvious to advertise only in mobiles. If you do it right from the beginning you can track both how many people clicked on your ad and how many people who actually bought the app.
Facebook provides the ability for companies to sponsor their own posts to improve the circulation.
There are several social networks but Facebook and Google are the two most effective ways to reach out to potential customers.