Instant Messenger: How does gtalk/yahoo messenger populate the contact list?

We are currently working on a small IM projec开发者_如何学编程t which pretty much works like gtalk and yahoo messenger. We came across a problem that puzzled us how gtalk/ym populate their contact lists. Given that the user has let's say more or less 500 contacts, both IMs seem to readily load the contacts pretty fast and already sorted. Here are my questions(referring to either):

1. Does it cache its contacts, like saving it in a file somewhere upon exit so that upon log-in it readily extracts the contacts and displays it in its contact list?

2. Does it always request for the VCARDS upon log in?

3. OR they have a VCARD push or whatever that simply updates the contacts' profiles (like that of their status [presence push - available, busy, etc...] )?