https://www.blockchain.com/explorer/charts/n-transactions
https://bitcoinvisuals.com/ln-capacity
Edit: another very concrete metric showing Bitcoin adoption is the daily global transactions fees paid to miners (in USD). This isn't the block reward, it is just the real costs (fees) that users are willing to pay to transact on the blockchain. The fact they are increasing implies increasing adoption/utility of Bitcoin: https://ibb.co/PgBm0t8 (note the log scale!) I like this metric as it's a 100% market-based valuation of Bitcoin's utility that is independent of the value of 1 BTC.