How do them service sites work like ning?

You know when someone creates an account right, and pretend they have a URL like so:
user_1.site.com
user_2.site.com
user_3.site.com

Do they create a database for every site?
(Different databases)
site_1_sql_db.sql
site_2_sql_db.sql
site_3_sql_db.sql

Or is it intermingled somehow?
(Single Database)
site_1_table_name
site_1_table_name2
site_1_table_name3
site_2_table_name
site_2_table_name2
site_2_table_name2
site_3_table_name
site_3_table_name2
site_3_table_name3

You can’t tell from the outside what they are using.
Both (and more) ways are possible.