I pay because I want to be the customer. If I am buying something, I am able to exert market pressure on the provider. If I am a collection of poorly-anonomized data being passed around from J. Random data mining company, I lose this (admittedly, largely nominal) agency.

I pay for email, I pay for hosting photos, and I pay for pinboard.

I paid for Pinboard in hope that the financial support will allow them to remain viable and keep the service open indefinitely. I've been burned too many times by using free services that eventually shut down when they realize they can't support themselves with ads.

I guess it all depends on how valuable the service is to you. For me, the one-time signup fee seems minimal for the value I get from a service that I use nearly every day. I've used many other bookmarking services, some of which have already disappeared like Gnolia. Pinboard is very stable, faster than Delicious, and the developers respond to requests quickly. I have no connection with Pinboard other than as a happy customer, but I regularly recommend the service and think it is worth paying for.

I use Pinboard to help collect data for many of my research projects and the archiving capability ($25/ year) is great retrieving pages that have already disappeared, that I want to go back and reference. Yes, I could use other free services and the Internet Archive Wayback Machine (which is amazing), but the speed, support, and reliability of Pinboard make it easily worth the cost. I prefer that services I rely on have a sustainable business model. Charging a very reasonable fee to provide quality service seems like a good model to me.

The original context is different and the tone is flippant but "If you're not paying you're not the customer, you're the product".

Considering how easy it is for ad-supported businesses to suddenly become unsupported and disappear I'm willing to give a nominal fee (~$8 forever) a shot.

I agree with skorgu, jfb and bengross. Personally I have used free bookmarking tools, RSS readers, blogging tools & "free hosting" for years, only to see them go belly up or the founders leave and the product is left to decay or suffer years of unnecessary feature additions.

It gets really old (and time consuming for those who don't have time to burn)

I recently bought Fever for my RSS reader and Pinboard for bookmarking both which I love (and are worth every cent)

Personally I plan on avoiding free SaaS based services in favor of paid or open source self hosted solutions whenever possible.

The alternatives which don't require your own server seem to support themselves through advertising. I consider viewing advertising a cost (and actually I consider it much more of a cost than the $5 I paid to pinboard.in). So, for me, there aren't any widely-available free alternatives.