Here is an interesting Database rankings survey on the db-engines site, which measures the general interest in the different databases, based on the criteria below.
This provides some insight into the trend in the adoption of these databases.
Database Rankings Criteria
•Number of mentions of the system on websites – From Google and Bing.
•General interest in the system – Using Google Trends.
•Frequency of technical discussions about the system – IT-related Q&A sites like Stack Overflow and DBA Stack Exchange.
•Number of job offers – Job search engines like Indeed and Simply Hired.
•Number of profiles in professional networks – Professional networks like LinkedIn.
•Relevance in social networks – Number of Twitter tweets.
For more information go to DB-Engines.