I'm from India. I signed up on Facebook in the year 2007 when I traveled to US and made acquaintance with people from abroad and wanted to stay in touch with them.<p>Between 2007-2009 I used Facebook mostly to stay in contact with friends who lived abroad. But slowly steadily the number of my friends who were signing up for the same reason was growing. BY 2010, I started seeing huge migration of people from orkut (the predominant social network in india) to facebook. Proabbly because enought people had moved to FB<p>Now a days, Facebook has already become a mainstream phenomenon. Telephone carriers, mobile phone manufacturers indirectly provide free advertisement for Facebook/Twitter by talking about their social features. Also, I keep seeing google ads of Facebook whenever i search for something that looks like a proper noun/ a name.
Many people have international networks of friends and relatives. My Facebook friends include people in Taiwan (both relatives and friends, some of whom I see in person once in a while through international travel), Singapore (Americans doing business there, whom I met elsewhere), Australia (friends from email lists), Canada (met as I met the Australians), and Japan (Americans living overseas whom I met on email lists). Friends tell other friends, and the network grows. As the previous reply points out, Facebook is advertised in many contexts by businesses, all around the world. All of these channels bring new users to the network.
I don't know about India, but Brazilians have a large online presence and love things that are popular in the US. Just take a look at the trending topics on Twitter and you'll almost always find something in Portuguese. I think that Facebook's worldwide dominance is what caused many to switch.