The word mainstream 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: 17
Highest Scoring Words With Friends™ Play In The Letters
Seven Letter Word Alert: (39 words)
amentia, ammines, amnesia, amritas, animate, antsier, artisan, artsman, artsmen, atresia, inmates, maimers, mantras, marinas, marines, marmite, martens, martian, martins, minaret, minters, mismate, misname, nastier, raiment, remains, remints, restain, retains, retinas, seminar, sirname, smarten, smartie, stainer, stamina, stammer, tramman, tsarina