How does one get started on creating a social community web application?

[eluser]shibuya[/eluser]
[quote author="xwero" date="1205600350"]First you got the have an authentication and an ACL system to identify users and guests and set the permissions for them.
From there it's up to you to add everything the community needs.[/quote]

So private messaging, image thumbnail creation, adding users to favorites, etc. need classes and methods created myself?