Short answer
Mostly not the parties' own pages. Party pages account for only 8.3% of political and issue ad spending on Meta in Britain, though 19% of the ads.
What the evidence shows
- Meta's Ad Library Report for the UK lists 90,565 advertiser pages that spent about £252 million on 1.51 million ads from October 2019 to October 2026. I classified 3,531 of them as belonging to a party.
- The biggest spenders in the whole library are charities and government bodies; the Labour Party's page ranks eleventh and the Conservatives' thirteenth.
- Labour spent £8.70m through 1,150 pages and put 38% of it through devolved, regional and local pages, against 24% for the Conservatives, whose national page took 58%.
- Reform UK has almost no sub-national tier (4.3% of its spending). Labour spent 14% of its money in Scotland, against 7.7% for the Conservatives.
Caveats
The data are lifetime totals, so the paper describes how party advertising is organised rather than what any one campaign did.