You are comparing to Facebook. Obviously Facebook has been trying to get into this market as well with their corporate offerings (which they've been surprisingly quiet about lately, suggesting that effort went nowhere). Maybe more appropriate for comparison would be their whatsapp team given that it is notoriously quite small given their enormous user base.
Slack is indeed just a chat app. There are many like it. Most of which look and feel very similar at this point. I administer a slack setup for our company and it's fine but it's nothing special. However, for what it does and what it cost, I'm not in a mood to replace it with something else. The hassle would cost us more and we'd not save a lot of money or gain any functionality that we need or indeed solve a problem we have.
We switched to Slack a few years ago from hipchat which at the time was very similar in scope, feature set, and cost. The reason we switched was that we wanted to get rid of bitbucket and some other Atlassian stuff (in favor of Gitlab, and later Github). I've also used stuff like IRC and even NNTP in the past, neither of which is appropriate for non techie teams. Lately, I've been considering switching to keybase which has a nice and easy to set up team component (I've actually set this up already). I'd probably go with that for new teams though it is still a bit rough in some respects.
Slack has awesome brand recognition but ultimately it doesn't have that many unique selling points beyond that. They've clearly grown by converting investor cash into customer acquisition. It's a common pattern with VC funded SAAS companies: compensate for a lack of unique selling points or technical edge with stupendous amounts of marketing and sales. If you think their hosting is expensive, their marketing and sales are likely way more expensive. It never was a proper tech company where things like algorithms, their awesome infrastructure, or patented stuff are the key things. It always was just another chat app done well.
Their hosting cost is quite high and suggests that they tend to throw money at problems instead of engineering talent. That's both fine and common for VC funded startups but it also suggests they will go through some lengthy rounds to re-architect internally and optimize their cost structure in the next few years after they IPO when shareholders are going to be obsessing about shareholder value.
From a technology point of view, they should indeed be able to run at a fraction of the cost but right now that's not a priority for them as they are very well funded and have a need to grow as fast as they can. Cutting cost through lengthy and complicated re-engineering projects is probably very low on their todo list and would be likely to just slow them down.
They are actually surprisingly middle of the road in terms of what they do. They do it well but when you look at their feature set there's nothing really that remarkable or unique. Their UI is alright but generic electron/react (?) which is notoriously not that fast but gets the job done. There are a lot of electron based chat apps out there and whatever your point of view on those is, slack is nothing special in that sense. Sure their UX is awesome and they clearly have some design hipsters running the show and obsessing over things like color schemes, logos, smileys, etc. But in the end it's just a generic chat app. E.g. Telegram, Signal, Facebook, Skype, Whatsapp, FB Messenger, (and Google's many attempts to compete with the Cartesian product of those) etc. each have very similar client side architectures and feature sets as well.
Sever-side they probably use Elasticsearch (which I'm well familiar with) and they seem to have a lot of centralized infrastructure and plumbing. From having used it, their search engine isn't actually that sophisticated or impressive. Clearly search ranking is not a huge attention area for them. Obviously a complicating factor is that they are running their stuff in multiple data centers across the globe. Adding to their complexity is enterprise needs for backups, auditing, security, compliance, etc.
Given their age and hipness, they probably bought into micro-services in a big way. That just means they run a lot of stuff that they scale by throwing more hardware at it. Over-provisioning is a great way to hide any performance issues. If you have dozens of micro services running in multiple data centers, things add up quickly. Add hosted data bases, search engines, queues, analytics, monitoring, devops, etc. to the mix and you are looking at some hefty hosting bills at the scale they are running it. Also many of their bigger customers probably insist on dedicated setups for them. When you grow rapidly, a lot of that stuff is just a side effect of Conway's law where you end up with a lot of moving parts because you have a lot of different teams.