What is worthy, bookmarking a website URL in large number of bookmarking sites or submitting it just to the top 25 sites?.
Before analyzing this subject let me also say that nowadays our clients are asking us, why don’t you provide another bookmarking service to 150 social bookmarking sites?.
But the question is what is the use of submitting it to another 50 low page rank sites?. If the Search Engines cannot index the low page rank bookmarking sites there is no meaning of having back links on such sites. But there is a possibility to generate traffic through these bookmarks. 
We should also note that all high page rank sites were brought up from low PR in a short period of time. At the beginning they might have been treated as low PR sites and now they have come up with a high PR as they became popular and fulfilled the Search Engine Strategy. There may be a delay on getting PR for some sites and of course it depends on the website, but all you know that how Twitter came up very soon.
By considering this, the low page rank bookmarking sites may get a PR anytime and it may raise up simultaneously. Once these sites become popular and get a high PR, the bookmarks within them too will be indexed automatically by the Search Engines, so you will now get back links as well.
As a conclusion social bookmarking to low page rank sites may not have much benefit now. But in future it may be much worthy in order to have your bookmark in front of all the posts.
