I have seen lists like this one - and I generally ignore them. I posted a list of 'Google purists' of bookmarking sites that have nofollow links AND they only list social networking sites that appear when searching the domain on (link: http://www.mydomain.com) Google within 60 days of posting a link.
Almost/ TheBiggest List of Social Networking & Bookmarking Sites on the Internet The list was submitted as an article to wealthy-writer (my blog) by a small SEO company. It don't think this is the biggest list - but it is probably the biggest 'moderated/edited' list. I know about 25 more bookmarking and social networking websites that 'never' appear when you search a domain's links on Google.
I have my own list of pure social networking websites, posted at the same website. I use these 28 sites regularly and many of them drive traffic to my website. I am thinking of adding another 10 to my list. I might add a few more to the other list - even though I did not post it originally.
I have tried a couple new ones - like www.yearbook.com but have no idea whether Google likes them yet. I have seen lists of social networking sites that were huge.
I once tried a press release company that gave me about 200 links, but within 3 months, almost all of them were removed when I did a web search. Of course, most of those links were to terrible websites, just spam sites.
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