I am creating a post to list all cloud-hosted alternatives to Firestore. All of these databases are scalable and serverless technologies.

1. Cloud Firestore

NoSQL Database built on Firebase platform

Amazing support for many different libraries
Incredible Default Realtime Support with Robust Caching
Wonderful Robust Security Rules for Backend Validation written in CEL (Common Expression Langauge)
Very Limited Querying
No support for any type of joins
Must handle all types of counts manually
Vendor Lockin, can't use outside of Firestore
Firestore Functions can do a lot in background
See here for my in-depth problems with Firestore (love/hate)
Price: Pricing based on usage

2. Dgraph Cloud

Graph Database using GraphQL known to be faster than Neo4j based on Badger.

Extremely Fast and Powerful
Has realtime support through graphql websockets.
Very Limited Backend Validation Security (Hopefully they are working on this), but can hack this with Custom Lambda Mutations (a lot more work) written in GraphQL Queries
No Nested Filters (same thing as filtering JOINS), should have this by end of the year, can hack with custom DQL queries
Has Data Studio like Cloud Firestore
Can Edit almost everything in Cloud Studio
Use my easy-dgraph js library for easily creating graphql queries
Best used with URQL for Caching
Will have storage usage in the future like Firebase
Must use external Auth Controls like Firebase Auth or Auth0
Price: $39.99 a month, or dedicated based on usage ($199)

3. Supabase.io

PostgresSQL using a JS Library with an online easy-to-use system.

JS Library makes any kind of queries easy to create, especially complex joins and filters
Uses Postgres's RLS (Row Level Security) policies for very robust backend validation
Has current storage bucket capabilities like Firebase
Has built-in Auth controls
Price: $25 a month, or dedicated based on usage

4. Hasura Cloud

GraphQL postgreSQL database

RLS for security
Needs Auth0 or Firebase for authentication
NHost.io built on top of this
Price: $99 a month standard to custom

5. Nhost.io

PostGreSQL with GraphQL built on Hasura

Adds storage and RBAC to Hasura.

6. MongoDb Atlas

GraphQL connection to feature rich noSQL.

Has Robust Join Capabilities for a denormalized database, but can be slow if not indexed
Many Languages Support, Functions, Triggers
Streams for Realtime Support via Websockets
Has a GraphQL api
JSON Schema Validation and built in RBAC
Price: $57 a month dedicated instance

7. Fauna

GraphQL connection for custom relational noSQL.

Realtime available using streams, limited
Built in ABAC, Custom FQL Rules for Robust Security Validation
Vendor Lock-in, can't view under-the-hood
Price: $22.5 a month, $135, $450, depending on needs

8. Neo4j Aura

GraphDb, basic graphQL, uses Cypher Language for everything else

Cypher includes RBAC and Robust Query Validation
Realtime support with external instance of Kafka (from my understanding), not built-in
Limited Documentation
GraphQL does not work out-of-the-box and needs a middlware server
Price: $65 a month to custom

9. Amazon Neptune

GraphDb, gremlin and SPARQL (latest technology!)

AWS (IAM) for ABAC Security
Lambda Functions (Java, Javascript, and other langauges) - Field Validation as well
Gremlin Websocket for Realtime
Limited Documentation
Easy access to all Amazon Services
Price: Custom pricing, probably starts at $450 a month

10. MariaDB Cloud / SkyDB

A Fork of MySQL management

Built-in User Management
MYSQL Validation Data Rules
Realtime Data Streams with Kafka (not built-in to my understanding)
Storage Options
Price: Custom Pricing, pay as you go, maybe $700 starting?

11. TigerGraph Cloud

GraphDB Restful Endpoint

Restful Endpoint
RBAC Built-in
GSQL Query Langauge for Security Field Validation
Kafka for real time streaming
Price: Pay as you go, dedicated, could get expensive, free version with credits

12. Nebula Graph

Chinese GraphDb with recent funding

RBAC
nGQL, openCypher for Security and Queries
Kafka or Pulsar
Nebula Graph Cloud in Beta

13. Redis Enterprise Cloud

Key Value Store that can be used as GraphDB

Rest API
RBAC
Pubsub
Price:Free Version, $7, and Flexible add GraphDB

14. HarperDB Cloud

SQL on JSON with Rest API

NewSQL, Microservice Architecture
Authentication Security (JWT and Roles), no field security that I can find
Pubsub websockets
Price: Free tier, per instance pricing

15. CockroachCloud

SQL on JSON

NewSQL, allows complex querying
Uses Changefeed to watch data
Price:: No Free tier, by the hour

16. Couchbase Cloud

newSQL noSQL allowing relational data

Spark Streaming, DStreams from DCP
Price: per hour, custom pricing

17. InfiniteGraph

GraphDB, claims massive scalability

Looks like GraphQL, probably websockets, documentation not available to free users?
Price: - Free version , $100 50gb

18. SingleStore

Elastic Cloud Database Relational SQL
Could not find specifics, has streaming capabilities
Price: - Credit system, free version gives $500 worth of credits

19. PlanetScale

Mysql compatible powered by Vitess

In beta, could not find real time data information or client side integration
Price: - low pricing for startups, cost per read / writes / storage

20. AstraDB

Apache Cassandra NoSQL database
DataStax Astra Streaming Technology
Price: - has free tear, price per usage

Honorable Mentions (No Cloud Service or Enterprise Only Cloud)

RethinkDB - JSON with joins, specialize in realtime data
Appwrite.io - noSQL, storage, rest api
GraphDB - Ontotext - RDF Db
Stardog - RDF GraphDB, has custom cloud version, but no free version
Apache Jena - RDF Triple Store

Let me know pros and cons of any you have used.
I will update the list if new features become available, so let me know.
Post a comment if you know of another cloud-hosted product (must be serverless), and I will add it here as well.

Cloud Hosted Databases DBaaS