The very latest series of major Google algo update Panda 4.0 rolled out a few weeks ago and they are following the same strict line as before on devaluing and penalizing low quality links and popular methods of link building and serp manipulations.
Specifically as guest posts became the most popular and an inexpensive method in increasing link popularity and improving search positions for a site, Google is now catching up and dealing with it. Earlier this year a major guest posting network MyGuestPost got penalized and even though guest post sellers are trying to spin it in a different light – the reality is that guest posts are no longer an effective link building method. In fact they may as well cause harm.
Especially the cheaper guest posts, from sites where a large portion of their posts are “sponsored” or promotional. There is still a thin line in there and some of the more natural looking posts like these, from higher quality websites should be ok, but they are also becoming more expensive..
But the good news is providing quality content or service and improving usability still works, as well as social marketing. However link building has to be done more and more carefully lately with long term perspective in mind.