Similar question was asked last year (https://news.ycombinator.com/item?id=12065355). Here was my answer then, which is about the same now, but the business has continued to grow:

I started https://www.pubexchange.com in 2013 and have been running it solo ever since. PubExchange is a platform that helps publishers establish traffic exchanges with other sites (similar to a social network) so that they can promote one another via widgets, social, and in-article links. It's been profitable since 2014 and I work with over 600 sites, including HuffPo, Refinery29, and POPSUGAR.