Come for the tool, stay for the network is a model used to describe the success of some startups. Basically, users come for a “single-player” tool and then stay because a valuable network grows around it. 

This is Chris Dixon's idea: 

I’m going to give two historical examples and leave it to readers to think of present-day examples (there are many): 1) Delicious. The single-player tool was a cloud service for your bookmarks. The multiplayer network was a tagging system for discovering and sharing links. 2) Instagram. Instagram’s initial hook was the cool photo filters. At the time some other apps like Hipstamatic had filters but you had to pay for them. Instagram also made it easy to share your photos on other networks like Facebook and Twitter. But you could also share on Instagram’s network, which of course became the preferred way to use Instagram over time.

The ‘come for the tool, stay for the network’ strategy isn’t the only way to build a network. Some networks never had single-player tools, including gigantic successes like Facebook and Twitter. But starting a network from scratch is very hard. Think of single-player tools as kindling.