Most social bookmarking links are permanent but most of them are also nofollow If you submit to digg or indianpad your story will get indexed within seconds but there are some that don't get indexed even in a week.
most of the times and a lot depends on the quality of content that you are submitting on these sites.
This is true that Indianpad and Digg indexes in seconds. Both are permanent as well. Most of the social bookmarking sites are permanent but if your link is outdated somehow then you link will disappear.
Yes, they're permanent. However for link building (SEO) it's a better idea to get "real" links. Social bookmarking sites are intended for driving traffic and getting your site indexed faster.