This is a proper ranking if you were to consider site traffic and popularity. Considering 3 month traffic ranks on Alexa.
#1 satoshidice.com
Traffic Rank: 141,892 Traffic Rank in CN: 147,337 (On a heavy decline, past 1 month traffic rank 377,179, probably due to just-dice, primedice launches)
#2 bitzino.com
Traffic Rank: 355,930 Traffic Rank in US: 502,297 (On a heavy decline, past 1 month traffic rank 792,559, probably due to bit777 launch)
#3 satoshibet.com
Traffic Rank: 460,124 Traffic Rank in BE: 9,627 (1 month rank 596,260)
#4 bit777.com
Traffic Rank: 646,229 Traffic Rank in US: 437,007 (We have most US players out of any site)
#5 bc-casino.com
Traffic Rank: 1,032,435
#6 bitcoinvideocasino.com
Traffic Rank: 1,068,517
#7 primedice.com
Traffic Rank: 1,139,314
#8 playt.in
Traffic Rank: 1,213,261 United States Flag Traffic Rank in US: 546,299
#9 luckybitcoincasino.com
Traffic Rank: 1,637,333
#10 strikesapphire.com
Traffic Rank: 2,689,574
#11 casinobit.net
Traffic Rank: 2,793,323
(not including just-dice.com as it's still not updated with current traffic and very new, it could land in #1-#5 range)
The ranking has been adjusted: Now Alexa traffic is leading, then facebook+twitter and then Top Bitcoin Sites votes and
the list is more like the above one now.