I don't think acquihires would count as R&D, acquisitions are booked differently.

My guess is that a large part of Twitter's R&D costs involve initiatives that don't relate to the main site's user experience at all: ads, live streams and mobile apps. Improvements on the sales/admin side for advertisers and content producers can be vital for revenue, but you don't see that if you're a Twitter user rather than a paying customer.

Same with apps: Twitter has Vine, Periscope, and who knows what else in development... But these don't improve the tweeting/timeline UX.