Your question is somewhat empty handed.
What kind of money do you have to put into this if any? Are you going to seek out investors if it comes to that?
Are you going to try and sell a product or are you looking for a quick get rich idea with no real overhead risk like Facebook?
Here is an idea that I am have always been looking for and it ties in with gambling.
There is a site somewhat similar to my idea that I use but is the only one out there that I have found. It is non sponsored site for MJ but just google the word COVERS.
Every gambler needs a site that give you info on past games,performances,stats,lines,totals,history.
This site has more than succeeded as they have sponsors out the ass. You can hit up sportsbooks, and even a MJ site for sponsorship advertisement.
You can setup fantasy sports leagues and charge which is all profit once you get the software to do it.
Fantasy Sports has never been bigger and it seems like everybody is always looking for a better model every year.
Gambling aids and fantasy leagues is where it is at.
Once you get enough sponsor income then you can start contests like the other site.
It will all be about getting your site out there and let word of mouth take it off.
This is offers plenty of stats for sports but leaves out the gambling side of it.
http://statsheet.com/
Fantasy Sports site is an easy payday as all you are in for is the software and maintaining updates on site and rest is profit.
Everybody uses Yahoo,CBS,or ESPN for fantasy and most people hate using them.