sticky
adj. Having content or features that encourage online visitors to linger at a website.

Example Citations:
But it was not realistic to expect a single website would become the predominant way people accessed the internet.

“We‘re not about building a portal,“ she said. SeniorLink was a resource for people to use.

Ms Schneider said many sites tended to focus on generating traffic and making themselves “sticky“ to lure advertisers.
—Ian Grayson, “Seniors corner a net mother lode,“ The Australian, May 2, 2000

Most so-called “top site“ surveys measure how many individuals visit a site....What they don‘t necessarily tell you is which sites are “stickiest.“ That‘s the new Web buzzword for sites that are especially good at keeping visitors engaged.
—Annette Hamilton, “Web Sites We Hate to Leave,“ Jesse Berst‘s AnchorDesk, May 1, 1998

Earliest Citation:
The 10 million members of the top three on-line services will see major changes starting the next few weeks.

The services are trying to compete against or embrace the burgeoning World Wide Web. They‘re also trying to make their services more “sticky,“ they say, by giving users something that keeps them from jumping ship.
—Kevin Maney, “New currents for on-line services‘ Net surfers,“ USA Today, July 27, 1996

Notes:
The sticky adjective is itself becoming linguistically sticky by latching on to different usages. Here's an example:

Genes released into the environment can replicate themselves ad infinitum. Indeed, some studies suggest that transgenes are particularly "sticky" — better at getting themselves around in nature than ordinary genes, possibly because of the viral and bacterial vectors used to engineer them.
—Michael Pollan, "Genetic Pollution," The New York Times, December 9, 2001

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