I get what you're saying but I'm not so sure -given it's team-centric (read: closed to outsiders) model - qualifies as network effect. That is I don't really benefit from anyone else on the "network" other than my team. So if your team doesn't want to Slack the friction here is so low (a need to setup your account) that the benefits (e.g., control, privacy, etc.) can be justified.
In addition the communication "niche" is MASSIVE. Communicate, as humans it's what we do. There's a reason why FB paid soooo much for WhatsApp, yes?
If add-on can make this product something between Slack and (the shitty implement of) FB groups, there's plenty of room for such a thing.
The difference is no companies were using MySpace, Usenet or GeoCities internally. Companies are using Slack. It's easy to switch when it's just chatter among unassociated individuals.
That's a good point about Slack being B2B. Thinking about B2B lockin, Slack has lockin to the degree that businesses are heavily invested in its unique features rather than just benefiting from improved communication.
Maybe fax machines make a better technical comparison. Email replaced their intra-company use in a few years. What kept them in use is that they are customer facing. Similarly, Skype and such replaced copper telephone lines for internal conference calls and in many businesses cell phones replaced landlines for intracompany communication and the phone system exists because it is customer facing.
