A few years ago, the Brooklyn Museum put on a Keith Haring exhibition, with a focus on his early career. There were videos of Haring at work, feverishly painting his way across an enormous scroll, and a room filled with drawings he illegally chalked in subway stations. But most stunning, at least to me, were Haring’s notebooks. They were displayed under clear cubes, their well-worn sheets pinned open for visitors to study.

The notebooks were sublimely surreal, filled with dogs crawling beneath bulbous U.F.O.s and penises ejaculating alongside concave cylinders that looked like nuclear cooling towers. By the time I first encountered Haring’s work as a teenager, his artistic legacy had been reduced to catchy imagery of colorful, blocky bodies hugging and dancing on T-shirts. But the notebooks showed what nagged at the artist, what motivated him. I saw someone so suspicious of government surveillance that he often wrote in secret code, someone obsessed with the subversive power of gay sex and someone working to merge his skepticism of capitalism with a deep-­rooted desire for fame and commercial appeal.

I left with an urgent curiosity about what sort of artifacts we would display a few decades from now, for future generations to discover. Our contemporary analogues to the personal notebook now live on the web — communal, crowdsourced and shared online in real time. Some of the most interesting and vital work I come across exists only in pixels. Tumblr, for example, contains endless warrens of critical theory about trans identity politics and expression, one of the few havens on the web where that sort of discourse exists. Many of the short videos on Vine feel as though they belong to an ever-­evolving, completely new genre of modern folk art. Some of the most clever commentary on pop culture and politics is thriving deep in hashtags on Twitter. Social media is as essential to understanding the preoccupations and temperature of our time as Haring’s notebooks were for his. But preserving materials from the internet is much harder than sealing them under glass.

Building an archive has always required asking a couple of simple but thorny questions: What will we save and how? Whose stories are the most important and why? In theory, the internet already functions as a kind of archive: Any document, video or photo can in principle remain there indefinitely, available to be viewed by anyone with a connection. But in reality, things disappear constantly. Search engines like Google continually trawl for pages to organize and index for retrieval, but they can’t catch everything. And as the web evolves, it becomes harder to preserve. It is estimated that 75 percent of all websites are inactive, and domains are abandoned every day. Links can rot when sites disappear, images vanish when servers go offline and fluctuations in economic tides and social trends can wipe out entire ecosystems. (Look up a blog post from a decade ago and see how many of the images, media or links still work.) Tumblr and even Twitter may eventually end up ancient internet history because of their financial instability.