Here's one of the Fidelity funds invested in Snapchat:
https://fundresearch.fidelity.com/mutual-funds/composition/3...
It's got $40 billion under management.
There's a document findable from there that lists their Snapchat investment as $10 million (Prospectus and Reports->Monthly Holdings report).
The number 1 holding of that fund is $2,192,901,654 of Apple. So they get roughly $10 million each time Apple pays a quarterly dividend.
I'd also like someone who understands the issue well to let us know whether these mark downs have any tax implications to begin with. I think it might sometimes be the case that marking down such a holding would allow recognition of a tax loss but I don't think that is going on here.
You're right that it won't be a large drop in the bucket. It's not news because it's a large drop in a bucket, it's news because its a valuation change on companies many here follow. Nonetheless, one could object to poor management if Fidelity didn't take this action, regardless of size.
Note - this only gets at the 'why now' part of the motivation. What it means for 1) their internal company valuations and 2) implications for shareholders without liquidation preferences is more interesting.
I think the mark down mostly does two things. It allows Fidelity to honestly inform their customers what they hold, and it signals to the startups that things aren't necessarily going the way Fidelity would like to see them go.
If these securities have been held for <1 year, their gain/loss goes into the short term bucket. So losses (in this case, writedowns) can offset the size of distribution, which is a bad deal for investors.
To a personal investor (e.g. mentioned in the world of WealthFront/Betterment), this is 'tax loss harvesting'.