No, Ethereum co-founder Vitalik Buterin is not a billionaire. While Buterin's net worth is estimated to be around $350 million, this does not make him a billionaire. The vast majority of Buterin's wealth comes from his holdings of ETH, the native cryptocurrency of the Ethereum network. At current prices, Buterin's ETH holdings are worth over $300 million. However, it should be noted that the value of cryptocurrencies can fluctuate greatly and rapidly. So, while Buterin is currently a very wealthy man, he is not technically a billionaire.