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. 2017 Dec 1;7:16743. doi: 10.1038/s41598-017-17135-1

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© The Author(s) 2017

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Time evolution of the new users and the activity period of the active users. (a) The number of new users per month versus time. (b) The histogram of the activity period of the active users. Each bin represents 20 days. The curves were shifted successively by one year so that they would fall upon each other if there was no change. The last curve 12.2012 was multiplied by a factor of 2 for easier comparison with the previous ones.