it was common in print media (Madinaveitia, 2018). Nonetheless, many sites and portals that attracted a large audience used it as a pricing method.

Another format emerged at the end of the 1990s, which has generated considerable controversy since it was launched. This was the pop-up, which the Interactive Advertising Bureau defined as ‘a type of floating format that appears on the user’s screen in an independent window or over the website content’(IAB, 2004: 12). Initially, they were considered a response to the sharp decrease in click ratios (Chaterjee, 2008: 53; Cho, Lee & Tharp, 2001), but they soon became a nuisance (Edwards, Li & Lee, 2001), whichtriggered the development of pop-up blockers.

At the turn of the 21st century, another remarkable phenomenon occurred: the explosion of search engines. Back in 1998, Goto.com became the first site to launchan advertising auction for keyword searches. However, although consideredgroundbreakingin that it launched the pay-per-click (CPC) system, it was perceived as lackingtransparencybecause it rewarded the advertisers who invested mostrather than content relevance (Cook, 2016). In 2000, Google launched the AdWords programme, fine-tuning it two years later and adding quality-based allocation, which allows ads to be ordered by combining not just the bid price, but also their popularity (Jansen & Mullen, 2008: 119).

However, until that point, as the IAB notes (2014: 8):

The ecosystem of buying and selling media was very simple: advertisers (and agencies that began to position themselves in the online medium) directly bought positions or impressionsfrom existing media. Soon it became evident that supply exceeded the potential demand and the available inventory increased daily.

In parallel, after the revolution of search engine advertising, the next milestone came from the social networks. These platforms gained strength during the first decade of the 21st century, and quickly came to the attention of advertisers, because they went beyond traditional media, offering the potential not only for accessing information but also for sharing and interacting more effectively. For authors like Alhabash, Mundel and Hussain (2017: 286) advertising in social media ‘is a piece of online content, designed with a persuasive and/or distributional intent through social media platforms that allows Internet users to access, participate, add and co-create’.

Facebook began working with advertisers in 2006, initially by inserting links (Text-links) and small images. Later, they implemented a system that segmented users according to socio-demographic characteristics and interests.

As of 2010, some platforms, including BuzzFeed or Mashable, opened up new options for advertisers to connect with the audience using sponsored content and native advertising (Cook, 2016). The IAB (2015: 2) defines it as ‘that which is integrated into the natural content of the page (...) allowing the brand to be present in the publication in a more harmonised way with the rest of the content than other advertising systems’. The format, which has considerable interest for brands and media, ‘blurs the division between editorial functions and advertising, which has generated some controversy’ (Carlson, 2015: 158).