I can only assume sqlfiddle has a business model/objectives1 in which higher viewership is a good thing. As such it is Stack exchange that is helping them by driving viewership to them! (or more reasonably it is a mutually beneficial relationship.)
I note a "donate" button on their site and adverts; so I put it to you that by driving traffic to them we indirectly fund them.
As such I would say that stack exchange shouldn't feel obliged to fund any site just because people post links to that site on stack exchange (of course a charitable donation would always be fine, but there are a lot of charities out there).
On the other hand if you actually use those sites and find them helpful then by all means donate.
1 Objectives monitory or otherwise. If I made a product and then a huge site basically said "This is awesome, go use it", I would actually be quite happy with that because it would help me achieve my objectives, whatever they are.