how do social bookmarking sites give you free traffic?

Norally depends from the social bookmark you refer to as regards the real system but generally they are a kind of search engine as the general features!!You add a link ,you can add tags or comment and so people who are connected on this social bookmark can search,find,mark,comment,discover your site!!You know a good position of your site or blog's post can be a source of free traffic!!If you count sites as digg,reddit,twitter etc..normally they are useful to give you free traffic as stumbleupon,delicious,etc..for websites...

Yes it does. You post and people read it. Very simple model. It increases your content exposure. The more you post, the more chance people visits it. well, provided it is good and interesting. A more complex way to look at it is social bookmarking sites tend to have higher keyword density and keyword relevance which in another words higher pagerank. Which in turn helps to bring your posted content on the search engines. Cheers.