Why do I need to trust facebook in order to chat with my friends there?
By using a web2 application (Facebook, Google, etc.), you are putting trust in the company to do the right thing with the data you give them (whether or not you want to). This is the same as sending a package through the mail. You're putting trust in "the messenger" until you can confirm your package reached its destination. Even then, you probably can't be certain that someone didn't open your package to look at (or modify) its contents.
For a lot of what you do online, you probably don't care too much. That's fine. But if you're like most people, you probably pause before entering your credit card or social security number (or whatever ID your country uses) into a website you've never heard of. On a decentralized blockchain, you don't have to trust a specific company or individual to trust it. That's why blockchain protocols are sometimes called "trustless".
This term is a bit misleading because, as Jonathan astutely pointed out:
Wouldn't I still need to trust a bunch of parties to chat with my friends "directly" in a web3 environment?
Yes, but blockchain protocols are designed to be "majority wins" -- so, even if you can't trust 49% of the network, you can still trust the network on the whole. This is why decentralization of blockchain protocols matters. If a single entity (or group of colluding entities) owns 51% control of the protocol, it is no longer decentralized and no longer trustworthy.
P.S. This is the first in a series of newsletters based on discussion with Jonathan. If you have questions about Koinos/blockchain/life, I reply to every email.