The main mastodon.social instance is funded through generous donations from users via Patreon! Server costs are also generally kept down by the use of different instances, which spreads out the amount of people on a single instance, lowers the number of users on an individual server, and distributes the cost between the people that run their own instances.
Because costs are kept low, and Mastodon is already funded through Patreon, you will never see ads on Mastodon. The only exception to this rule is when companies make individual accounts on Mastodon, and post their ads to the Whole Known Network and local timeline. However, these "ads" are like any other post on these timelines, and blocking the account will remove the posts from your view. Companies that create accounts on Mastodon and advertise excessively in the timelines are also usually banned from using it.
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