Frankie Vallee was the lead singer for the Four Seasons, who had quite a string of chart hits in the early-to-mid 60's (Silence is Golden [the original hit], Rag Doll, Dawn, Let's Hang On [to What We've Got], and probably a dozen others I can't think of right now). He went on to a solo career which included a hit version of "Can't Take My Eyes Off of You".

I would include some Bee Gees songs as examples of being sung in falsetto.