Database Design For Social Network apps
One table for all users.  Period.  End of discussion.  (This is a common question; the answer is always the same.)

When you get to a million rows, we can discuss some other issues and solve them before you embark on the road to a billion.

What is best for project X?  If it will have a billion rows, you will need a database expert to tweak whatever underlying system you are using.  Any system can do it.  But it won't be trivial.

Focus on getting to a million rows.  Reconsider all of your design decisions at that point.  Any decisions made today will not survive to 1 billion.

Would you like me to repeat myself all over again?