Kinda helps that they only employ something like 30 people and run their own servers. Their overheads are likely to be extremely low.
Compare this with Reddit, with 2000 employees and a monstrous AWS bill.
They said they have 12 million paying users. 30 employees is nothing - even if they all make 500k/year. Is everything else simply servers and bandwidth?
Mostly bandwidth and a rack space, you can do a lot of work with one 2U server these days.