At Primal, we index Nostr data for the sole purpose of providing discovery features to our users. 

Our entire software stack is open under the MIT license. That means that anyone can use our software to do exactly the same thing we are doing. I believe that our indexer is the only open source indexer on Nostr; please correct me if I am wrong. 

It should be understood that Nostr is a radically open network. The social graph and all published data are publicly visible by everyone. Running an indexer is not hard, so I don’t think that “selling this data to advertising agencies” will be a competitive business. Anyone who wants to have this data will have it easily enough.