They're getting cracked because they're not paying attention to their security.
But start-ups are all about the most buzz you can generate in the shortest time. You need to get that product out the door ASAP because your competitors aren't going to wait for you to build your secure system first. After all, you're not in the business of security, you're in the business of connecting up the most people and building your community.
*Added because even I thought I was starting to sound like a lean-startup advocate