Is there any sense to put some work of creating 3-5 fake accounts to land on front page by voting on pages like upcoming.pl or folkd.com. Does such links have big seo potential?
They have little to no SEO impact so don't waste too much time. Social bookmarking sites are good for driving direct traffic, not for link building and SEO.
As zexy said they aren't going to do much for you SEO wise. If you can get an article to the front page of a digg or reddit however it will help you hugely not only in terms of traffic, but in terms of SERP's for that article.
The actual link from the site will not have much/any effect. The follow up links that you'll receive (bloggers & other webmasters linking to your content just because they like it) is enormous. Exposure's exposure.
Posting links on Digg has helped me, but BARELY enough to notice. I get about 2 clicks through it a day and it's a week old.
Main problem of social bookmarking websites is nofollow tag in outgoing links. If you will post your site to dofollow bookmarking websites you will get good effect on SEO.