Very simple.

1. LinkedIn is THE #1 Job Board, hence attracting millions of job seekers and service providers alike
2. LinkedIn has almost a billion professional users (900 million to be exact), and it's adding 3 every second!
3. LinkedIn is using its marketplace platform to cater to 3% of its users who are buying in any industry and at any given time.
4. Easy to grow its marketplace by tapping into its 900 Million user base via events and emails marketing