Because while, according to CNN, he was outspent on TV advertising by all the other major candidates, he ended up with more votes than his predecessor—with a share of almost 40 percent.
Part of the reason is that the average age in the country is 23.5 and so 'millennials' are estimated to be 46 percent of the electorate. 'Free facebook' deals from the country's telcos mean unprecedented social reach: In total there are an estimated 44.2 million active internet users, 42 million active social-media users, and 36 million active mobile social users in the country. Facebook reported 268 million conversations about the election—leading it to call the 2016 Philippine polls the ‘most engaged’ elections in Asia Pacific.
After the elections, Duterte's spokesman thanked 14 million 'social-media volunteers'—across Facebook, Instagram and Twitter. His online video reach was 21 million views, compared to just 3 million for the next best candidate. Duterte's Facebook page has gained 3 million likes since November 2015. And the candidate dominated all online polls.
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The Du3 phenomenon: How social media won the presidency in the Philippines | Digital | Campaign Asia