The PPL and the C++11 concurrency libraries are not really direct substitutes. PPL offers a higher level, task based approach to concurrency and can be both easier to work with and more efficient than managing threads directly using std::thread or similar threading libraries. PPL also includes things like parallel algorithms (parallel_for_each, parallel_transform, parallel_reduce, parallel_sort, etc.) and concurrent containers which are not included in the C++11 concurrency libraries.
It's also worth noting that the PPL and Intel's TBB (Threading Building Blocks) are designed to share the same interface to a large extent and TBB is supported on Linux so there is some cross platform portability when using the PPL. Intel, Microsoft and NVIDIA have a joint proposal for a parallel algorithms library for future versions of the C++ standard (C++14 and beyond) which are based on the PPL/TBB.