They've done a good job upping revenue over the last few years (somewhere north of a billion dollars/yr in 2014). And, IMHO, done so without the resulting ads being too obtrusive.
Their problem isn't that they can't make money. It's that their valuation means investors expect them to make a ridiculous amount of money. So all revenues are plowed back into R&D.
That's a bit of an accounting smell, isn't it? Obviously Twitter should have some costs other than R&D, like labor and servers. Even if we say the rest is going to R&D, that's kind of a black box. It doesn't prove that they aren't simply recording reasonable recurring costs correctly while hiding bizarre business-killing recurring costs in R&D.
They're a publicly traded company with a "big-4" auditing firm to make sure that doesn't happen. The law is very clear on what counts as CapEx and OpEx. Obviously, companies occasionally outsmart (or team up with) their auditors to defraud the public, but the vast majority follow GAAP and succeed or fail on the merit of their business.
It's OpEx either way, right? Ideally the firm would be able to point at x, y, and z innovative and profitable new services, and say that's why we spend so much on R&D. It's not clear to me that Twitter can do that. So, an investor would be justified in suspecting that not all R&D expenditures are valid.
Their problem isn't that they can't make money. It's that their valuation means investors expect them to make a ridiculous amount of money. So all revenues are plowed back into R&D.