Some SEO companies used to use private blog networks to rank themselves and their clients in Google, However I believe that Google knows everything that happened on the internet and they decided to de-index all of those private blogs.
What does it mean ? means those blog will no longer be in Google search result. thus the links on them will have no value. How to know if a site has been de-indexed by Google ? easy, just search Google for the site domain name ( for example search for example.com ) , if no result in Google that means they got de-indexed One of the main players in Private Blog Networks was "Build My Rank" , And they got it as Google had de-indexed many of their private blogs. Thus they had to close their business as they announced in their blog on Mar. 22nd 2012. Another network (ALN ) lost 5,297 Domains in 1 week ( out of 17,657 ) more
How this affects SEO companies in Singapore?
Well, If you were one of those who used a SEO service that uses private blog network then you you will need to check your rank and keep an eye on it. as this not the end of the story I expect more to come.
I personally noticed 3 sites where in high position in the 1st page of Google for a very competitive keyword and now they are no longer there.
Some webmasters also got Unnatural Link Warnings from Google through webmaster tool and some have been penalized.
So, what next ? Google is working hard to provide quality search experience. and they watch everything on internet. so just give them what they want , Quality and Value . Just my 2 cents.