After seeing what happened to Flickr (both the product and the team) after Yahoo bought it, combined with Slack's crazy growth, makes me think a sale is unlikely. Especially to a big company like MS.
But Microsoft is the most likely buyer, it's been on a spree with integrating best-in-class enterprise products/productivity tools into Office 365 (Sunrise, Accompli, previously Yammer). Slack fits into that mold, though the sale price would probably be orders of magnitude higher than those acquisitions (likely upwards of $10 billion, possibly much more). At that price tag, it might be untenable.
Not sure what Slack's medium-longer terms goals are, honestly. Their valuation is so high that only megacorps can afford them, but there's still enough risk that public markets might be skeptical (their revenue growth is astronomical, but they haven't been generating revenue for THAT long). In the interview, Stewart mentioned market saturation eventually limiting their crazy growth numbers, but I'd imagine they have a ways to go, given that the total enterprise market has to be well more than 200,000 total seats. Exciting times for them.
To what extend that's literally true or it's just that he's just working with some of the same people and tools, I don't know, but it would certainly make sense.
In much the same way Slack rose from the ashes of Glitch, Flickr did from Game Neverending, the predecessor to Glitch.
http://gaming.wikia.com/wiki/Game_Neverending
>Although development of the game was later shutdown in 2004, the tools built for GNE later evolved into Flickr, a widely-hailed photo-sharing service. Occasional signs of this legacy are visible, such as the '.gne' file extension appearing in Flickr's URLs.
http://techcrunch.com/2012/11/15/as-flickr-co-founder-butter...
>Whether or not the next venture is based around imaging services, it looks like it will have a social element: “We have developed some unique messaging technology with applications outside of the gaming world and a smaller core team will be working to develop new products,” he writes.
200k users could be achieved by signing up 3 or 4 megacorps (obviously at enterprise rates far lower than $10 per user) and having a business on that scale that doesn't make you a unicorn, even if you have a decent product and profitable business model. In fairness to Slack, the revenue run rates not mentioned in the article are more impressive, though I haven't seen how much they're spending to acquire those users.
Skype had well over 50x the user base of Slack - achieved over a similar timescale - when they sold for a similar valuation back in 2005. Frankly, I was far more impressed with how circumspect Butterfield was about his startup's prospects than the growth figures.
Microsoft would want to buy Slack for the same reason they spent $$$ on best-in-class products like Yammer, Sunrise, rebranding Acompli as Outlook. Slack's revenue is incredible for a startup, but a drop in the bucket for MSFT (for now). More immediately, Slack represents an entry point for Microsoft to upsell Slack's customers to other Office365 services and Azure services.