I'll explain this quickly because I've been asked about it a couple of times today:

Mastodon has no ads. Its only source of funding is donations.

You could donate to the open source project itself – a fine thing to do. But at the moment, with the huge influx of new users, the funding is better off going to the instance you are using.

Each instance usually publishes a donate/patreon/crowdfunder/etc link. If you go to
[your instance url]/about
you should find information about it.