Which social media platform was the first to introduce political ad transparency tools that showed users who paid for political advertisements and how much they spent?

Social media has dramatically transformed how political campaigns are run and how governments communicate with citizens. From Barack Obama's groundbreaking digital strategy in 2008 to the constant Twitter updates from world leaders today, social platforms have become essential political battlegrounds. This trivia question focuses on a pivotal moment in the relationship between politics and social media advertising that changed campaign strategies forever.

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Facebook (in 2018 after the Cambridge Analytica scandal)
Twitter (in 2017 before the platform banned political ads)
YouTube (in 2016 after the US presidential election)
LinkedIn (in 2019 as part of their professional transparency initiative)