So, YouTube had 50 million users and 65 employees in 2006 when bought by Google for $1.65 billion. Instagram 27M users and 12 employees today for $1B.
Though Facebook paid much more on a per-employee basis ($83M vs. $25M), it did pay only 12% more on a per-user basis ($37/user vs. $33/user). So maybe this deal isn’t as extraordinary as one might have thought.
At this $-per-employee rate, Twitter would be worth $5.2 billion.