Ask HN: Why are there not many successful link centric startups?

Very few companies have been able to make a breakthrough in this area even though the volume of people sending links, saving links is huge. Why is this so?

Many have seen success isolating and focusing the experience of photos, videos, etc. But not links.

Could it be the case that Facebook, Twitter, etc are actually link sharing companies? Or is there something inherently different about the link as a content type?