Matrix is a protocol, not a "service". Chose a server you trust, or selfhost (and use E2EE).

Matrix the org was hacked.

The matrix.org infrastructure was hacked. Yes, this is obviously a problem (though their recent blog post does help restore some confidence).

But saying we shouldn't use Matrix because of this event is like saying we shouldn't use IRC because Freenode was hacked. Or we shouldn't use XMPP because HipChat was hacked. Not to mention that Matrix has sane E2EE support so folks who were using it didn't have to worry about their conversations being broken (and E2EE-by-default for Matrix is on the horizon).

That's like complaining the yaml.org site was defaced, so we shouldn't use yaml.

AFAIK the CI server was also compromised, so it's more serious than some website being defaced.

the CI server that was compromised was used (mainly) for running tests rather than builds. we don't believe that any builds were compromised. https://matrix.org/blog/2019/05/08/post-mortem-and-remediati... has the full details, but yes: just because the project's infrastructure got breached doesn't mean that the protocol or wider network was affected: it wasn't.