Note: I do not know the internal mechanics of Kickstarter (some comment states that it does not work as explained), so I will work on Rickstarter description by the OP.
The funds are managed and held by Rickstarter but they are not owned by Rickstarter; it works as a scrow service.
For each $1 of cash directed to funding a project Rickstarter accrues an obligation (either towards the donor or the project to be funded)1 of exactly $1; in the books there is neither profit nor loss. At some point of time RS will get its cut from that $1.
It is similar to getting a $100.000 loan from a bank: you are not any richer because with the money comes the obligation to pay it back.
A point of interest is when Rickstarter gets its profits. The basic rule is that Rickstarter can only put into the books its income when it has earned it and can be reasonably sure to get the money.
For example, Rickstarter could stablish in its service terms that it gets a 5% rate from any donation, and in this case it could be reasonable to consider a 5% of any received money to count towards the profits immediately. If there were a time for the investor to retreat his money without penalties, then the profits should only be accounted after that period. Or Rickstarter could apply a 5% rate to the money directed at projects that get enough funds, and in that case it could only write his profits in the book the moment the project gets that limit. In case that Rickstarter allowed people just to pledge the money, it would be unwise to count the profits before people had actually paid the money....3
Now, direct taxes4 on business are almost always directly linked to profits, not to the size of assets, so Rickstarter's taxes would be linked to its cut of the money (minus all expenses, of course) and not related to the size of assets that it keeps in scrow.
A different (and a lot more complicated) issue would come from regulatory obligations: there could be laws mandating that if Rickstarter manages more than $X assets then it has to start making audits and comply with other rules. But, while following those rules can have a cost, that would not make them "taxes".
1The specific details of which kind of account would be convenient to reflect this obligation is beyond my knowledge of accounting.
2Here the prudence concept comes into play, you cannot consider that you got your cut until you are reasonably sure that you have earned it.
3This can become very complex. For example, in the case of a pledge RS would probably write as an asset "the $1000 SJuan76 owes us", but at the same time if there are doubts about SJuan76 ability or willingness to pay it should also anotate a liability of "the $1000 (or $500) SJuan76 might default" so it would not be count (at least totally) towards profits.
4There may be taxes based on activity (e.g. sales tax) or related to some assets (for example property taxes on the buildings owned by the company), but I think you are not asking about those.