Can anyone please explain why social bookmarking isn't considered effective?

I have found a lot of claims regarding social bookmarking being of no use in the modern-day SEO, anyone please suggest why is it so? And shall I stop making efforts for the same as I am doing it for my website?
Social bookmarking is valuable and effective.
Social bookmarks show the value of your content to the search engine algorithm.
It helps to improve the ranking, credibility, and authority of your site.
 
Why do people even do social bookmarking when they don't get any traffic from it and when the links are of nofollow?

It's not effective.
 
I strongly disagree. Linking from most social bookmarking sites may actually harm your website.
I might consider them being ineffective but could you explain how could it take a negative turn?
 
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