Since it was introduced in late 2004, Google Scholar has rapidly grown to become a widely used tool for finding and assessing the impact of academic literature. The database still suffers from noise relative to its competitors Scopus and Web of Science but it has broader coverage, especially in the social sciences and humanities and is open access. As the database developed, Google have periodically added new information sources to the database. This resulted in a rapid growth in estimated citations of articles in the early years. However, it now seems that the database has matured. The following graph shows the growth rate of citations to my research in the previous 12 months, measured monthly since 2009 for Google Scholar in blue and a bit more intermittently for Scopus in red. I have also fitted exponential trend lines to the two series:
Initially the growth rate of Google Scholar citations was very high and very erratic. But the month to month variation in the annual growth rate has reduced drastically over time. By contrast, the growth rate of Scopus citations has been much more consistent, with a slow rate of decline in the percentage growth rate over time. Interestingly, the two series have also converged to a common growth rate of 17-18% per year. So, it seems that Google's database is now as mature as Scopus is. This doesn't mean that Google is now as high a quality data source as Scopus is. It isn't. But large revisions to citation counts or additions of large new data sources seems to be a thing of the past.