A Google Webmaster Help thread has a message from Google's John Mueller that offering an incentive to link to your site in exchange for unlocking features is most likely against Google's webmaster guidelines.
A site owner asked if it would be acceptable to give his users of his site an "upgraded listings" in exchange for linking from their web sites to their profile on his site.
In short, you link to us, we give you more features for free. It is free, but we want a link in exchange for the feature we are giving you.
Google's John Mueller said this is a bad idea. He said he would "strongly recommend not making PageRank-passing links to your site a requirement for any kind of interaction on your website."
Back in 2007, we talked about ethical link building incentives and in 2009, we asked if incentives are paid links. Back then SEOs were split, some saying yes, it is like a paid link and some saying no.
Google clearly here is saying, these types of incentives are a form of trade and payment and make for an unnatural link for the purpose of manipulating PageRank and potentially rankings and thus is against their guidelines.
Here is John's full response:
I'd strongly recommend not making PageRank-passing links to your site a requirement for any kind of interaction on your website. Links placed like that are generally not natural links, not the kind of links that our algorithms want to find. Past that, not all businesses or people have real websites, it seems like it would be a bit unfair to block them from being able to use your site to its fullest (and in turn, if they end up loving it, recommending it to their friends & business partners). By all means, make it easy for users to recommend and to link to your site, but don't use that as a requirement.
What do you think?
Forum discussion at Google Webmaster Help.
Image credit to BigStockPhoto for carrot and stick