Even though Mnesia was developed by Ericsson, I'm not aware of any of their products using it. Are there any?
Rabbitmq used to... think it still does. Which NF's it goes into I don't remember off the top of my head but I would bet tons of like messaging stuff.
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 :)