Hacker News Reader

There's a tax credit for claiming your work is R&D:

http://en.wikipedia.org/wiki/Research_&_Experimentation_Tax_...

This creates an incentive for companies to find ways to report their work as R&D. (Not implying that Twitter is defrauding or that their work isn't R&D, just stating why it might be useful for a company to "invest in" R&D, especially if that is work they wanted done anyway.)

r&d most likely relates to product development and isn't academic research. By listing it as r&d they can probably write something off with the tax man.

Things such as their discovery tab, targeted tweets, trending, search and user search most likely involve (Or involved) reasonable investigation.

I've no idea what source of R&D Twitter does, but you can't necessarily judge them based on what they blog about or release papers on, plenty could be going on behind closed doors.