Crowdfunding backers
Post it the day the campaign closes, while backers are still checking the page. Show the total, the thing they funded and when it will arrive. A second, shorter thank-you when rewards ship keeps the goodwill going.
Creators and their patrons
Monthly supporters rarely hear thanks beyond an automated email. A ten-second clip with what their money paid for this month (a new microphone, a studio day) is personal without being awkward.
Clubs, choirs and campaigns
Members who sold raffle tickets, sponsors who paid for the kit, neighbours who signed a petition: one video for everyone, shown at the next meeting and posted in the group. For a charity appeal with donors, the donor thank you video page goes further.