Not sure how it is today, but in the 90s and early 00s when I worked at Ericsson Mnesia had a number of systems that it was used in. There was interestingly 4 DBMSs developed at Ericsson in the 90s and 00s. Mnesia and NDB Cluster as mentioned and two internal DBMSs, TelORB and DBS, I had interesting discussions with all of them and worked with DBS before developing NDB Cluster. We were all situated within 5 minutes walking distance :)