R&D is probably almost all going to be salaries for engineering talent. It takes a lot of people to run your own end to end solution for 500m users. Remember, they are doing everything from software development to running data centers.

They also work on stuff like [Dropbox Paper](https://www.dropbox.com/paper), a markdown capable minimalist Google Docs alternative.

I’m not 100% sure but I believe this is also called R&D because it had different financial/accounting meaning which affects how spend/revenue are reported.

They can be considered expenses that are reported on taxes, and also since there is risk of no or poor ROI knowing the R&D component of the budget helps adjust for risk in modeling future revenue and expenses.

That also suprised me- I know that they spend a great deal on optimizations to reduce backend cost, improve uptime / reliability, etc., but I don't see much on the customer side myself...

You underestimate the amount of engineering effort required to simply run a fast-growing highload service.