His Internet fame originally came about because of his status as the (late) lead developer on Tumblr. Marco frequently writes insightful posts on Apple products that are linked by John Gruber of Daringfireball, who reads his blog attentively. This means his blog posts show up on HN and echo across the entire Apple-focused blogosphere. Apple bloggers are a great audience to have, because they are used to promoting paid products. So Marco has a huge blog readership, and his frequent mentions of his experience with Instapaper in his writing create a huge awareness of the product.