Private venture-backed startups like Slack and Airbnb are investing in other startups. Here's where their money is going.

Mike Segar/Reuters

• Privately held companies like Slack and Airbnb have started investing in other venture-backed startups through corporate venture capital funds.
• Corporate venture capital has been popular with tech companies in the past, with major players like Qualcomm and Google each spinning off at least one venture arm of the business.
• Together, corporate venture funds from private startups have invested more than $2 billion since 2013.
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Some venture-backed startups don't want to miss out on the next wave of unicorns, so they're turning to an old funding strategy.

Workplace chat app Slack was one of the earliest privately-held startups to use its funds to back other startups in 2013. In the years since, Airbnb, Stripe, and Coinbase have also started corporate venture capital funds of their own. In total, these four funds have invested more than $2 billion across 76 companies, according to Pitchbook data.

Read More: Tech VCs are squabbling over a popular type of funding for startups that one prominent investor calls a 'nightmare' and a 's**t show'

Corporate venture capital isn't a new phenomenon, and tech giants like Google and Qualcomm all have one or more existing corporate venture funds to take big bets on emerging technology.

"Being a part of a large company that has incredibly talented engineers, executives, and a global presence means there's a lot of money and resources that can be delivered to early-stage companies and entrepreneurs that accelerate adoption of the product or service. It also helps them not make mistakes others have made," Valo Ventures founder and former CapitalG founder Scott Tierney told Business Insider. CapitalG was formerly part of Google's corporate venture arm, Google Ventures.

We looked through Pitchbook data to determine the top corporate venture capital funds from privately-held startups. Here are the top four: