As u might have studied google runs on page rank technology.
page rank involves
a.markov chains
b.stochastic matrix
c.eigen values

to compute a page rank of a page is by usin formula
prev page rank/no of links out of the page.

A Page can become Authority (no of page that refer u) and hub (outward link from ur page)

AS RICH GETS RICHER

google has to improve the performance of page rank by using pages from communites ORKUT which has big community can help in their research
to improve ranking of website

I don't think the Orkut community can help Google in any way... in fact is giving them more problems than anything (check http://news.google.com/news?sourceid=navclient-ff&ie=UTF-8&rls=GGGL,GGGL:2006-11,GGGL:en&q=orkut+brazil)

To improve the performance of search results (or something like it) Google launched Co-op (http://www.google.com/coop)

please read this
A densely linked focused sub-graph of hubs and authorities is called a community.
Over 100,000 emerging web communities have been discovered from a web crawl (a process called trawling).
Alternatively, a community is a set of web pages W having at least as many links to pages in W as to pages outside W.

Wasn't it the case that Google semi-owned Orkut from the beginnings, and that its creator was their employee?

S Suresh: You mean that Google is using Orkut infrastructure (communities) as a test to improve the performance of his webcrawling and pagerank? It can be possible, but I just don't see Orkut as the best test field.

Phillip: Yes, Orkut Büyükkökten was a Google employee that created Orkut in his 20% time.

Yes you are right Philipp. I think that Orkut is probably the worse service that Google is involved with. I am told it is down a lot of the time and the interface is absolutely shocking.

Pau,Philip,Sam do have any hint abt any communities that has more users
  

According to google paper published by Larry and brin
from them-"
Collect the top t pages (say t = 200) based on the input query; call this the root set.
Extend the root set into a base set as follows, for all pages p in the root set:
   add to the root set all pages that p points to, and
add to the root set up-to q pages that point to p (say q = 50).
Delete all links within the same web site in the base set resulting in a focused sub-graph"

this root set has high rank that google bot starts with -why?
they might have access to pages which are not indexed by crawler

there is also called base set

root set(A) base set (B) (A intersection B) gives the unique pages.