The nature of bookmarks is that you generally want them in long-term storage.  Relays are not long-term storage, they relay events from one user to another.  In general, they are ephemeral storage.  Howevr, NIP-98 allows integration of nostr identity with long-term storage systems.  That has a capacity of over 1 million times the relay network.  Things like tinygem, and all sorts of micro apps can be built on top.