Back in February Duncan described Spanish language Sonico as “the biggest social networking site you’ve probably never heard of”. At the time, Sonico had raised to an Alex rank of 167 after just 6 months in operation. And with over 8 million users, it was also a top 50 site in several Latin American countries including Colombia, El Salvador, Bolivia, and Mexico.

Fast forward about four months and Sonico has signed up almost 17 million users (although its Alex rank has dropped a bit to 297). The Argentine startup has also decided to raise its first round of institutional funding – $4.3 million from London-based DN Capital and private investors that include OLX’s Fabrice Grinda and Alec Oxenford, and FON’s Martin Varsavsky.

Sonico is among several Spanish and Portuguese-speaking social networks that have established themselves in Latin America – including Orkut, Hi5, and Wamba – but face increased competition from mainstream American social networks like Facebook and MySpace.