First of all, since I always like paying special attention to beginners, I'd like to explain exactly what dofollow social bookmarking sites are. Social bookmarking sites, in order to prevent spammy submissions, usually add nofollows, which are basically an instruction that the Google bot gets telling it not to pay attention to the link on their site towards yours. Therefore, the link does not enhance your site's PageRank, which is one of the most, if not the most important factor to get top spots on Google.
Are these links useless then? The answer is no. They are still useful if you want traffic, publicity for your site and reader interest, among other things. However, there are dofollow social bookmarking sites out there. Right now, we're talking about smaller websites, not the most famous, but submission is useful. It won't replace a quality SEO and social media marketing strategy, but it can be an interesting part of it.
Here is a very complete list of some dofollow social bookmarking sites which I've put together. You might find more spam than usual here, and remember that these sites tend to appear and disappear fast - you might have to really go through the list and re-check it once in a while - but they are still a valid option:
BackFlip
Blinklist
Blogoria
Chain-Marking
Chipmark
Connotea
Current
Diigo
Dizzed
Ebaum's world
FeedMarker
Folkd
Gozoof
Jim's Free Links
Jumbomark
Kingtop
Kwoff
MyLinkVault
MyPIP
Oyax
Oyax
Planete Blog
Plime
So Force
Social Article
Tagza
Tamilers
Taqza
astoreseo
bibsonomy
blogmark
booklandonline
bookmark-tag
chainmaster
citijournalist
contentpop
countingbiz
crowdfound
delmaster
experts-views
eyoorl
ezinesocialmark
firstbookmark
firstdigg
firstwebonline
freelinkvault.
mainlinks
manualsocialmarks
memfrag
multiplysocial
mysocialmarker
mywebdesktop
qiinga
reebiesup
royallisting
seoblisting
socialbuilder
socialearth
sociatectmark
spotlinking
stingclip
surfurls
technoratitags
theclaimid
thelifelogger
themagicmenu
tweakonews
ugotreview
voteboat
way2marks
wedelia
xilinus
yomashnews
yucoffee
zangkok
So how do we discover nofollow links anyway? Although I'm usually a Google Chrome user, for this I recommend using the Mozilla Firefox browser, because it enables you to install what are called add-ons. If you search for 'firefox nofollow addon', you will be able to install a number of programs that mark nofollow links with red highlighting - a very useful trick for any webmaster. Whenever you go into a website which is all marked in red, you'll know that links placed there are not useful for SEO purposes. On the other hand, if there's no red highlighting, you're good to go.
Other good ways to get dofollow links is by submitting work you write to certain PR (or Press Release) sites. While traffic on these isn't generally huge, the dofollow links are valuable: just the same situation as with the dofollow social bookmarking sites we listed before... These PR sites don't value quality content as much, so it's fine to submit less newsworthy or informative items, such as announcements about your business, which - let's face it - might be a little bit less interesting to a general public.