(Reuters)-AmericanGreetingsCorpreceivedago-privateofferfromagroupledbyitschiefexecutiveandothermembersofthefoundingfamily,valuingthecompanyatabout$580million.AmericanGreetings,whoserevenuehasshrunk17percentinthelast10years,hasbeenforcedtospendmoreonadvertisingasitstrugglestostayrelevantintheFacebookage.Thecompany,whosebrandsincludeAmericanGreetings,CarltonCardsandGibson,hasalsobeenhur...