Asana's Rosenstein said there was a learning curve involved, but even "super-not tech savvy" people were converted once they started using the product.
"It's about getting people over that initial hump, and then they never look back," he said. "At some point it will seem very old school to send an email. At the best companies in the world it already does."
Companies currently using Asana include Dropbox, Uber and Airbnb.
Slack is another company that has successfully attracted significant investment in this space, and is often referred to as the billion-dollar company you've never heard of. Last week the messaging and project management start-up announced that it had raised $120 million in a financing round, valuing it at $1.12 billion including the fresh capital.
CEO Butterfield said that Slack had two significant advantages over email: firstly, it's more transparent as everyone on your team can see what others are working on. "You get an ambient awareness of what's going on across the organization."
Secondly, the information is searchable. "Rather than having all that information locked up in individual inboxes, it's a shared resource which develops over the course of months and years."
Investor Selegenev highlighted that there was now quite a lot of competition in the sector.
"The market place is definitely crowded now," he said. "A lot of the companies in this space are very similar, just have slightly different angles."
'What the world needs'
Another company keen to get in on the act is note-taking software Evernote. Earlier this month it rolled out Work Chat with the aim of improving collaboration across teams at work.
"The inbox makes people dumber. The inbox is the king of bad ideas," CEO Phil Libin told CNBC at the Web Summit conference. "Email is something that needs to be significantly reduced."
He stressed he didn't want to kill email—because it's good for certain things—but that in a perfect world people should only receive five emails a day "max".
The issue is that email is based on physical mail—which is great for love letters, but not for much else. "Email can still do that, but it's terrible for talking about work things. Email needs to really be diminished," Libin added.
Evernote, Slack and Asana all use a freemium model, with free basic products and premium versions that cost extra.
Evernote Business costs £8 ($13) per user per month and Slack Standard costs $6.67 per month (in 2015 the company is launching Enterprise for between $49 and $99). Asana is free for teams smaller than 15, after which it starts at $50 per month per team.
These software-as-a-service (SaaS) business models, with the potential for reliable recurring revenue, could mean big bucks for the company that wins in this space.
"If someone builds a proper solution to this problem, it's the next Facebook," Selegenev added.