I'm currently developing a social networking site that may, hopefully, in
the future need to scale out to multiple, maybe even many, database servers.
Will it happen? Who knows, but I'd like to be prepared for this now, so
adding additional servers will be quick and somewhat painless. Even if it
never happens, at least I'll learn something new.

So, this is what I'm thinking....

a database for basic user info
a database for profiles
a database for messages
and so forth

If the site gets to the point where it needs to have additional servers,
there will be the option of having the databases on their own servers which
could each serve as masters in the future for replication if needed. The
database of basic user info would be replicated across all servers so the
info can be used for joins (to get usernames when querying messages for
example), since cross database joins require the databases to be on the same
server.

Since multiple database servers and replication is new to me, I thought I'd
get some input from you guys here. Does this sound like a good way to go?
Or, is there a better way to do this? I am aware that I could just put
everything into one database and replicate it out to many servers, but I
figured that since some databases will be used far more often than others,
it would be better to separate them out so that the most used databases
could possibly get more slaves than the less used ones.

Thanks in advance for any input, suggestions, etc.

Steve