The word efficaciousness is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 29
Highest Scoring Words With Friends™ Play In The Letters
Seven Letter Word Alert: (51 words)
accuses, affines, caisson, caseins, casinos, cassons, cession, ciscoes, cocaine, coffees, coffins, concise, concuss, confess, confuse, cosines, cousins, cuisine, effaces, effuses, encases, eosinic, fancies, fescues, fiancee, fiances, finesse, fission, focuses, fucoses, fusions, iciness, incises, infuses, ionises, nicosia, oceanic, offence, offense, offices, osseins, science, sconces, seances, seasons, session, sinuses, snuffes, success, suffice, unifies