Transaction volume has always been a difficult metric. For one thing, most transactions are likely to happen off-chain. That can occur in a centralized system like Coinbase. This has always been a possibility dating back to the first exchanges like Mt. Gox.
More recently, Lightning Network means that decentralized off-chain transactions can play a bigger role in Bitcoin's total transaction volume. Lightning involves two parties setting up a contract (Bitcoin has supported them from day one). This contract defines rules that allow the parties to update a private transaction (without counterparty risk) until settlement on the block chain is required. At that point a transaction is published. That transaction may represent one, a handful, or hundreds of thousands of intermediate transactions. There aren't very good ways to determine how many, and this will tend to make Bitcoin's transaction volume ever more opaque over time.
Exchanges are starting to support Lightning as a method to move bitcoin in and out.
Second, absolute transaction count doesn't take coin value or age into account. For example, I can just flip the same 0.001 BTC back and forth to myself rapidly. That would add many transactions but wouldn't reveal much of value.
An alternative metric is "bitcoin days destroyed" (BDD). BDD equals the value of the coin being spent multiplied by its age in days, summed over all transactions in a given period:
https://bitcoin.stackexchange.com/questions/845/what-are-bit...
When BDD spikes you know that either old coins are being spent or that lots value in younger coins is being spent. That metric might be more economically valuable. There's a realtime chart here:
https://blockchair.com/bitcoin/charts/coindays-destroyed?int...