We used Slack at my last job but we were encouraged to stop by the person's desk for any matter that requires immediate attention. I sat near our CTO and sometimes I saw many engineers standing around his desk trying to debug a production issue. This resulted in knowing other people's name/face (which I realized after I joined my current company).
At my new company we use Slack for pretty much everything. If there's a production bug you're encouraged to @here on the dedicated channel and someone would take a look at it. There's no one-to-one interaction to debug it. We had our holiday party last month where I introduced myself to some of my coworkers and once we started talking we realized that we have chatted on Slack but never saw each other even though we were all working in the same office. I never associated the engineer's face to the name.
I do think Slack is a powerful tool in modern work environment but people shouldn't be too dependent on it.