Does private social bookmarking network help to increase pagerank? how does this different from link farm or link wheel? What are the differences between private social bookmarking network, link farm and link wheel?
I wouldn't count on it increasing your page rank, but it will help potentially bring traffic to your site and help with indexing speed too.
Depends on what you mean by private. If it's private as in only certain submissions or people can submit or be accepted then it will work decently.