It was also a big day for thousands of our supporters, who'd been inspired by our social media campaign to contact their MPs to ask them to vote in favour. Months of effort to encourage our social media followers to become activists had paid off.
The campaign proved something that an alarming number of people in PR still seem not to understand: a big social media audience shouldn't be an end in itself.
It's easy to set and meet targets if you care only about numbers, but no-one ever won a campaign by recruiting supporters who don't do anything. For a charity such as Stonewall, which was founded on grassroots activism, engaged followers are vital. They save money and help make campaigns more effective. They're the followers we should be counting.
Our engaged followers clearly made a difference. Failed lobbying campaigns on everything from tuition fees to the smoking ban indicate that MPs aren't often swayed by grandiloquent press releases, petitions or sign-and-send postcards. We wanted MPs to listen, so encouraged our followers to contact them with personal stories underlining the need for marriage equality.
Judging by the number of MPs who said their vote was swayed by these thousands of stories, our supporters can be proud that they made a difference.
This underlines the cost-effectiveness of building and mobilising an engaged audience. Unlike our opponents, we can't afford to print millions of postcards, or hire glitzy studios to film little-seen vlogs.
Our supporters wouldn't tolerate our spending their donations on scattergun tactics. Tweets cost nothing and we have never paid Facebook for promotions. We have not wasted a penny in achieving our goals.
Practitioners will argue for years about how to measure the effectiveness of social media campaigns. Our engaged followers played such a decisive role in this campaign that the lesson is clear: big audiences are worthless if no-one is listening.
Andy Wasley is media manager at Stonewall.