Social Bookmarking links are do follow or no follow links?

Hello,
Social Bookmarking links are do follow or no follow links?

Dofollow links are an HTML attribute that is used to allow search bots to follow the links. If a webmaster is link to your site with dofollow link, search engine bots and people can follow you. They pass on link juice and really benefit your website to get a high Page Rank

Social bookmarking sites generate a lot of traffic.