You can submit any page in social bookmarking. Any such new link only has positive effect, as such websites are well regarded by google for the most part.
If you submit homepage of a commercial site to digg or newsvine, they will be very prompt in banning your user. Create content that you think has potential to go viral and submit it to social bookmarking sites.
You can submit any page of your site..page one time.....yes it is useful in ranking and link popularity...
you can submit any website , the main page or deeper page to those social bookmarking sites. That's why they even exist I suggest you to use this big list of social bookmarking sites: http://www.basic-directory.com/social_bookmarking_sites_list.html