Dofollow bookmarking sies list?

[zeus]

Google+ 9 NA NA
Pinterest 9 38 43
Reddit.com 8 45 29
Tumblr.com 8 49 31
Examiner.com 8 2,563 8,298
Google.com bookmarks 8 2,467 NA
del.icio.us 8 5,059 9,342
Citeulike.org 8 26,874 118,022
Blog.friendfeed.com 8 46,709 456,897
Digg.com 7 1,460 3,244
Slashdot.org 7 3,994 5,016
Fark.com 7 5,047 3,579
Diigo.com 7 2,991 12,987
Newsvine.com 7 16,358 58,509
Technorati.com 7 60,822 247,269
Scoop.co.nz 7 78,325 113,935
Pearltrees.com 6 7,966 12,275
wwwhatsnew.com 6 11,128 25,541
Hubpages.com 6 2,454 7,619
Dzone.com 6 4,074 11,994
Metafilter.com 6 5,368 5,110
Folkd.com 6 7,458 47,714
Blinklist.com 6 58,028 506,480
Pulsepoint.com

Google+ 9
Pinterest 9
Reddit.com

These are most social networking sites. How come they provide do-follow links?