Which types of orkut communities gets popular?
The comercial communities like promoting a site with the orkut communities is a bad idea. Actually, most of the users of orkut doesn't like to go out of it and spend their time hanging with their friends. They even don't like to surf other sites stories. So, they search the social communities like any fan club of any celebrities whom they like most. More the fans, more the member joins the community.
Need of active community member.
Just getting more member joined in an orkut community is useless because your members must have to be active. Active members can only the medium to spread your community to more and more users. The communities based on Organisation, Charity, Spirituals, language,etc.. can be called as dead communities, here members joins the community to just match his/her identity, religious, his schools, to pay respect, others.. where they join just for name of joining. No reactions and nothing at all.
What type of communities should be created?
Communities related to the topic entertainment should be created to attract the active orkut users toward it. Topics based on any TV serials, Reality show, any orkut applications and any celebrity fan clubs attracts most of the active users.
Only creating an interesting topic is not finish :
You need to update your community content daily by which the users can recieve the daily updates on that topic and discuss on those topics.
More spices can be added by putting daily poll over which mostly users like to vote and participate daily.