Most of the good spontaneous conversations I've had at work were previously on Slack anyway, for a couple of reasons:
- You're not interrupting someone / taking up their whole time. If you have a thought, they can wait until they're free to reply, and if you're both free at the same time you can have a good conversation. But you were still able to write out and communicate the thought.
- More people can join and provide input. I can always talk about things with a couple folks on my team, but it's super valuable to have someone from another part of the company pipe up and say "Hey, what about this" without having to consciously think of inviting them (and interrupting them, see the above point - the way to do this in person is to have extremely large "user group" meetings that are low value for their opportunity cost for most of the attendees).
- I can easily find people in other parts of the company / work across teams because teams tend to be in physically separate locations. It's super easy for me to run something past someone who works in the other building; it would take about 10 minutes of my time total to go there and back, and then I'd have to hope they're at their desk.
- This is something of a mixed blessing, but conversations are logged, so you can reference them later. To the example you give about chatting about things after a conference, I tend to write up what I've learned or ideas I've followed on and put them into Slack pretty quickly, with the expectation that we'll have a conversation about them at some later point but not immediately.
What I've been trying to do is to expand all of this and drive even more conversations to Slack. (To be clear: have the conversations in existing public channels, not in DMs, and avoid tagging people in. Also, do not expect people to read any particular conversation. Slack has a pretty good search feature, so if they ask about something later, link them to the previous discussion. I'm not at all advocating that you take up people's time with pings on Slack.) We do also have occasional video calls for higher-bandwidth conversations, but I've found they work best when they're structured as accelerators to an async written conversation and not as discussion forums in their own right.