Great question!
Scroll down to the bottom!
https://givebutter.com/givingmastodon
"Mastodon Inc
A 501(c)(3) Public Charity
EIN 92-3333630"
Also here.
https://joinmastodon.org/about
Search for EIN:
"Mastodon, Inc. (EIN 92-3333630) is a 501(c)(3) non-profit entity in the United States that supports the growth and operational capabilities of Mastodon, including being able to receive tax-deductible U.S. donations and in-kind support."