One Large Table or Multiple DBs?

Hi,

What is efficient approach toward storing a web application's user
data. How do applications such as basecamp, SalesForce or QuickBooks
online store their data? Is it in one DB with huge tables each record
having a user account's foreign key or do they create a separate
database for each of their accounts? Which one is more efficient? My
guess was in one large DB with large tables.

Thank you,
Mike