Many of the artists I discovered via Spotify / Google Music make pennies from these sites. The truth is that unless you're buying an album (1) or other merchandise, you're not contributing significantly for the artist's survival.
I was chatting with Brock Van Wey last week (aka bvdub, my favorite choice of background music when programming) and he was describing how challenge it is to stay in business nowadays. I naïvely thought these apps were helping those folks...
I for one will try to buy more albums, even if digital, of the bands I listen to all the time.
(1) directly from the artist (e.g. via bandcamp). Google Music, at least, doesn't really give you the mp3 but only a protected file that can solely be played in their software.