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.
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
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.