Enrique Orduna-Malea et al. have self-archived "Empirical Evidences in Citation-Based Search Engines: Is Microsoft Academic Search Dead?"
Here's an excerpt:
The goal of this working paper is to summarize the main empirical evidences provided by the scientific community as regards the comparison between the two main citation based academic search engines: Google Scholar and Microsoft Academic Search, paying special attention to the following issues: coverage, correlations between journal rankings, and usage of these academic search engines. Additionally, selfelaborated data is offered, which are intended to provide current evidence about the popularity of these tools on the Web, by measuring the number of rich files PDF, PPT and DOC in which these tools are mentioned, the amount of external links that both products receive, and the search queries frequency from Google Trends. The poor results obtained by MAS led us to an unexpected and unnoticed discovery: Microsoft Academic Search is outdated since 2013.