It's not a secret that paid links as SEO technique doesn't meet Google requirements, moreover, unnatural link building violates the Webmaster Guidelines. Google keep fighting against paid links.If formerly amount of quality paid links was main factor for ranking, currently such criteria as engagement with users and content shared in social media are taking into account as well. Successful webmaster should understand not just SEO concepts but marketing techniques also. Matt Cutts suggests that webmasters forget about SEO and create quality and useful sites for users. Nevertheless link trade stayed lucrative business niche. First of all, quality backlinks are still significant criteria for page rank. The point is that not every paid link is considered to be unnatural and harms the site. See Matt Cutts Answers Below in Video.
1) Quality backlinks on trusted resource within the industry.
Let's see. First of all, the link should be relevant, for instance, if you supply casino apps, don't place it at website about animals. Second is authority. Such factors as domain authority and page authority are essential, but the most important thing is that host website should be trusted, get traffic and not look like link directory. Thus, traffic will bring clicks, besides, quality sites don't offer a lot of place for links. So it gives competitive advantage and good relation of Google.
2) Sponsorship backlinks.
If the company is a sponsor, it places their links on sponsored site. This type of links is considered by Google as if a sponsor had a right to require a link. Another area for paid links is social media. First, as it was mentioned above Google likes social references. Content shared in social media increases position in SERP and attracts traffic. Consequently, paid likes, followers, and fans are in demand. Social media development drove increasing amount of networks that unit advertisers and account owners. Besides, social media provided with work software developers that create software for likes generation and so on. To sum up, in view of recent events Google requires backlinks to be corresponded with high standards in order to pass page rank and content to be "social" to improve SERP position.