Instead of ads, the users will pay for the infrastructure by paying tiny amounts of bitcoin when downloading (uploading) from (to) a Nostr relay
Some of us are working on the tech to make that a reality. High frequency, trust-minimized, bitcoin payments allowing any new relay to accept these "Cashu Spilman channel" payments with minimal setup
There is also nostr:nprofile1qqspqflat0pmtegvnqqdfz7g4nau9yxcnwzhcl8p24e22uzgcnq9trsppemhxue69uhkummn9ekx7mp0qyg8wumn8ghj7mn0wd68ytnddakj73h7976 's notary system: Anyone who writes a note can spend a tiny amount - which is then "burned" on chain in an efficient way which is easy for Nostr consumers to verify - and then consumers (relays and nostr clients) will give higher priority to nostr events which have been "notarized" like this. This won't directly fund the infrastructure, but it will discourage spam and therefore might decrease the cost of running Nostr infrastructure