Well, one is a platform provided specifically for work related communications (again, like your email). Also, read the above linked article which discusses wiretapping:

>employers are given an exemption for calls made “in the ordinary course of business.” Courts interpret this to mean that employers can eavesdrop on all business telephone calls but cannot listen to or record messages it knows are personal.

Also:

>ECPA also applies to audio monitoring of the workplace. Employers can install recording devices in any location that is used primarily for work. But employers may not conduct audio recording of nonworking areas such as cafeterias, break rooms, or locker rooms. In practice, this means little because employers are not required to notify employees that they are being recorded and employees are unlikely to discover the hidden microphone.

So there is an obvious legal distinction between communications which are intended to be work related and those which are not. In your cafeteria example, no, they should not (assuming this article is the entire story.) However, your slack messages are not considered the same as a cafeteria conversation.

Again, I'm not a lawyer and have zero real knowledge here, but I think it's silly to expect privacy in your work provided messaging system.