One or more tables?

Hi.

For an example let me say that I have a big (over 1 million) user
"base". Then every user does a lot of inserting/updating of data.
Would it be better to create different tables for insert/updating for
every user or  would it be better just to have one big table with all
data (tables would have of course the same columns, ...). How do you
cope with this kind of things?

1.example (1 enormous table)
tablename (id, user_id, datetime, some_data)

2. example (a big number of tables)
tablename_user_id( id, datetime, some_data)


Thank you.

Kind regards,

Rok