Parallel programming is hard. The industry leaders hope to convert the hard problem of using parallelism into the easier problem of using a parallel library. Yet, we know little about how programmers adopt these libraries in practice. Without such knowledge, other programmers cannot educate themselves about the state of the practice, library designers are unaware of API misusage, researchers make wrong assumptions, and tool vendors do not support common usage of library constructs.
We present the first study that analyzes the usage of parallel libraries in a large scale experiment. We analyzed 655 open-source applications that adopted Microsoft's new parallel libraries -- Task Parallel Library (TPL) and Parallel Language Integrated Query (PLINQ) -- comprising 17.6M lines of code written in C#. These applications are developed by 1609 programmers. Using this data, we answer 8 research question and we uncover some interesting facts. For example, (i) for two of the fundamental parallel constructs, in at least 10% of the cases developers misuse them so that the code runs sequentially instead of concurrently, (ii) developers make their parallel code unnecessarily complex, (iii) applications of different size have different adoption trends. The library designers confirmed that our finding are useful and will influence the future development of the libraries.