Flies were never my favorite, although I used to do them quite a bit. They were a little hard on my shoulders and what not, so when I dropped them I didn't miss them. If you are looking for more mass, then do heavy pressing on bench and dumbell bench (inclines also). My chest has gotten bigger and stronger doing heavy bench presses, much more so than it ever was doing flies.
If you look at Defranco's westside for skinny bastards, on ME day, he has one working up to a heavy triple on bench press, then doing around 4 sets of dumbell bench presses afterwards. This worked wonders for me.