The word posterities 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: 14
Highest Scoring Words With Friends™ Plays In The Letters
Seven Letter Word Alert: (52 words)
erotise, persist, pesters, petites, pettier, pieties, pietist, pissier, pitiers, poesies, poetess, poetise, posters, potters, potties, presets, pretest, priests, protest, protist, reposes, reposts, respite, respots, resteps, retests, ropiest, rosette, rosiest, septets, setters, sitters, soirees, sorties, spirits, spotter, spriest, sprites, stereos, stipite, stories, streets, stripes, tersest, testers, testier, tipsier, tipster, tiptoer, tiptoes, treetop, tropist