A lot of them are sales people. LinkedIn as a people business needs a lot of sales reps and account managers to be in contact with Recruiters and other people involved with selling .
It is not just big, it is mega big. WhatsApp had world dominance and their product had to deliver instant messages, including photos, videos and other features over crappy internet connections and they managed to build, manage and sustain an amazing consumer product with a handful of people, so 16k for an online job board is laughable and embarrassing. It's just a website after all.
16K people spanning the whole globe is not bad .