It's too broad a question and I'm not sure it's even at any significant scale. There might be the odd tech savvy youngster who would be capable and have any interest in setting up such a site, but it requires a huge amount of effort to make it all functional - a barrier that will stop 99.99% of people from doing it. There's also a range of ways to do it, such as using forum software like this - creating threads where people submit pool bets and then keep track of the outcomes in a more secure place like google spreadsheet. You're very unlikely to find many instances of small scale sites getting popular enough and feature rich enough to be holding balances, as there is a lot of infrastructure required behind the scenes for that - but generally, yes, they would be hackable in all sorts of ways.