E-mail is the best asynchronous narrowcast online communication medium

I think e-mail is the best way to communicate online, to a group smaller than the general public, when a real-time response is not required. I think this for a few reasons:

E-mail is (almost) universal

E-mail is, if not truly universal, the contact method that's closest to being universal among netizens. It's so much so that most other contact methods require their users to sign up using an e-mail address. Multiple times I have wanted to contact an offline friend/acquaintance and found e-mail was the only communication medium we had in common.

E-mail is decentralized

E-mail, by its very nature, is not restricted to a single provider or piece of software. Because e-mail uses open standards, e-mail providers and clients are numerous. This means if one e-mail provider or client becomes defunct, e-mail as a whole will persist. Compare this to centralized communication media, where if the central service goes down, the medium goes down with it.

E-mail is longevous

E-mail has been around since 1971 and continues to see widespread use. It's robust enough to have lasted 50-odd years and, I believe, to last many more years into our future.

Potential downsides of e-mail

Despite its advantages, e-mail is not perfect:
E-mail files are usually not encrypted, meaning a malicious or insecure e-mail provider could cause one's private e-mails to be used undesirèdly. This can be countered to a degree by using PGP encryption, but most netizens do not use it. The effects of PGP-encryption are also reduced by the fact it is only useful when both an e-mail's sender and recipient(s) use PGP.
It has also been pointed out new e-mail providers could have difficulties if the major e-mail providers refuse to connect to them.

In conclusion

E-mail is not a perfect system, but I believe it is the best asynchronous online communication system out of the options available.