What type of database (nosql type) is used in messengers?

Question

NoSQL

All NoSQL DBMSs are divided into several categories:
Key-value stores
Column Family (Bigtable) stores
Graph Stores Graph
Document Stores Document-oriented DBMS

Answer 1

No
sqlite
In places for levelDB cache

Answer 2

There is also a DataGrid - it's all taken together, and even in memory (Hazelcast), and even with disk storage (Apache Ignite). Here you have everything at once and Key / Value, and objects (Document), and SQL, and distributed, and BigData / BigTable.