An old professor drilled it into my head, “the word ‘data’ is plural. It is a count noun, so you never say ‘data is’ only ‘data are’.” However, most people say ‘data is’ because it just sounds better (the hip-nerds at FiveThirtyEight thinks so and their Twitter poll agrees).
I suspect that the computer-age is changing how we talk about data. As data increasingly becomes a part of every day life, it seems natural to use the word ‘data’ as a mass noun, synonymous with ‘information’. For example, “Data is increasing at an incredible rate.”
This is simplistic preliminary evidence, but notice in the graph below that usage of ‘data is’ and ‘data are’ starts to converge in the early 1980’s when personal computers are gaining traction in the market. Like most tendencies in language and culture rules shift to align with use. While I much prefer ‘data are’, the people have spoken (over the last thirty years), ‘data is’ works in some situations. The question is will the masses continue to blindly use data as a mass noun in situations where it is clearly countable? Undoubtably, yes. I’m afraid to say data is doomed to be the mass(es) noun.