My understanding is that the current charitable donations are backed by a grant from CEA, so in the narrow view this money was absolutely going to a charitable endeavor of some sort, regardless of the actions of manifold users. This is pretty strong evidence towards zero, in that the actual money being donated was pre-earmarked for charity.

I think two better ways of looking at manifold for charity are firstly that you are buying the privilege of allocating charitable dollars. So, while the money was counterfactually going to a charity, it was not necessarily going to the charity you selected. Depending on how narrow your circle of empathy is, the degree to which the donation is counterfactually valid changes. If it's very wide, for example if you don't discriminate heavily between the available charities, then the donation is not very counterfactually valid at all. If you're suspicious about this whole charity business but are really into haskell, then the donation is counterfactually valid, I would imagine a small fraction of the available funds would have made their way to the Haskell foundation. Secondly, I think it's also helpful to look at it as something like Freerice, all of the available money was always going to go to charity, and in that sense, you individual moment to moment actions create no additional donations on the margin. However, it creates a sort of fiat charitable value, it's not tied to any real counterfactual change but everyone agrees to accept that it sort of is and reasons about the actual number accordingly. So, you may not be creating counterfactual donations but you are creating a community of people who are invested in charity and charitable donations. In the longer run, this leads to more interest and visibility and probably more total donations. For example, when I talk about manifold I can say that I'm competing for some fake digital currency. However, that currency is backed by hypothetical charitable donations. If you're worried about people viewing manifold as a site for degenerate gamblers, then being able to point people to the charitable donations page is a large benefit. I think the Hansonian framing is that this consecrates or makes more sacred "mana" as a fake currency. The ultimate consequence is that the charity arm of manifold benefits both manifold and the actual charities, and so continuing to engage with it may have more impact in the longer term.

So, I think the two questions you need to answer are firstly, how wide is your circle of empathy, how much do you care about the right to allocate capital, and secondly, what is your view on the time value of charitable donations. If your views are very wide and very short term, I argue that it should move closer to zero. If they're very narrow and very long term, it should move closer to one.

I'd personally place it somewhere between 0.2 and 0.3

Large donations are definitely counterfactual: Imagine buying 10x the size of the charity grant pool worth of mana, and then donating it. Obviously the money's coming from you, no matter how it gets routed.

Small donations that add up to a large donation are counterfactual if they cause Manifold to secure additional grant funding(presumably using interesting betting activity as a justification), and not if Manifold doesn't.

So small donations need a "chance that Manifold goes bust" or "chance that Manifold doesn't get extra funding" or "chance that Manifold grant-funding stops appearing useful to donors" multiplier.

Counterfactual of any charity getting the money, or of a specific charity getting the money? If it's a specific charity and not one already popular with EA I'd say close to 1.

If it's any charity I'd say about 0.4. See below market which gives only a 68% chance for the charity program to continue once the current grant runs out, which I think is a minimum (but not sufficient) requirement for the donations to be counterfactual. https://manifold.markets/jack/when-the-current-500k-grant-for-man-9aa6ef92353a?r=QQ

When the current $500k grant for Manifold For Charity runs out, will the charity program continue?

64% chance. From https://help.manifold.markets/manifold-charitable-donation-program Manifold for Good has received grants totaling $500k from the Center for Effective Altruism (via the FTX Future Fund) to support our charitable endeavors. As of the time of this writing, every unit of mana is fully…

0.15? It seems lots of conditions need to exist to exceed zero: Manifold survives to give out 500k on the initial grant, doesn't get a new grant for this purpose, decides to continue the problem, doesn't devalue the 1:1 ratio. I'm skeptical of Manifold's pathway to revenue, so assign probability accordingly.

Galaxy Brain: If Manifold commits to destroying* undonated charity money, then the existing charity grants become counterfactual donations for individual donors.

[*] "destroying" could mean "donate to the most useless charity possible"

My personal answer to this question is adapted from @Sailfish. The value of manifold donations is redirecting money towards charitable causes I value. If all the money donated through manifold would go to charities I would also donate to the counterfactual value would be 0

In order to find a precise answer for myself I took the list of the charities from https://manifold.markets/charity and gave each of them a factor between 0 and 1. A factor of 1 means I would pay 1$ to redirect a dollar away from this charity towards a different cause and a factor of 0 means I would pay 0$ to redirect.
It turns out I would pay 0.46 cents to redirect a dollar gifted through manifold towards my preferred causes. So my personal counterfactual value is 0.46.

I donated mana worth 1000$ and will reduce my donations by 460$ to account for the counterfactual value of my donations.