In legal terms, Citizens United wasnt about the content of the Hillary ad - it was about who paid for it. Zuckerberg gave all that that money to nonprofits, and the non-profits did electioneering with it, as enabled by the citizens united ruling, letting in billions of special interest dollars. Mcconnell said this was a good thing for free speech at the time, but I bet he is changing his tune now.

"The case began after Citizens United, a conservative non-profit organization, sought to air and advertise a film critical of then Democratic presidential candidate Hillary Clinton shortly before the 2008 Democratic primary elections. Broadcasting the film would have been a violation of the 2002 Bipartisan Campaign Reform Act, which prohibited any corporation, non-profit organization, or labor union from making an "electioneering communication" within 30 days of a primary or 60 days of an election, or making any expenditure advocating the election or defeat of a candidate at any time. Citizens United challenged the constitutionality of this law, and its case reached the Supreme Court."

"Senator Mitch McConnell commended the decision, arguing that it represented "an important step in the direction of restoring the First Amendment rights".[3] By contrast, former President Barack Obama stated that the decision "gives the special interests and their lobbyists even more power in Washington".[4] The ruling represented a turning point on campaign finance, allowing unlimited election spending by corporations and labor unions, and setting the stage for Speechnow.org v. FEC, which authorized the creation of "Independent Expenditure Committees", more commonly known as Super PACs, and for later rulings by the Roberts Court, including McCutcheon v. FEC (2014)"

https://en.wikipedia.org/wiki/Citizens_United_v._FEC