Determining susceptibility to frequency analysis
Whatever parameters you pass in will affect the resulting hashes.  If you compute only H(Firstname), then two users with the same first name will lead to the same hash even if they have different surnames.

P.S.  Consider letting users simply have a name so that you respect the vast populations of people out there whose names do not conform to the anglophone forename/surname model of names, or other problematic oversimplifications.