What makes these tools succeed at least for the short term, is they fuck with the organizational habits of people. Their landing pages are huge cues to get more organized, which people love. People love the idea of getting their shit together and becoming organized once and for all.
The challenge these apps are facing is creating a habit loop. You need a cue and a reward, but more specifically when replacing a habit (email) with something new, you need to replace either the cue or the reward. In between those you have your tool. Without this change people just go back to their old habits. In the case of Inc the reward is "thanks for sharing that link, you’re awesome!" and the cue is "I need to send this to my whole team". Currently everybody immediately reaches an internal mailing list or enters in a few autocompleted emails when they encounter this cue, the urge to share with team members.
Now what Kippt, their original product did, which was clever, was creating new cues. They heavily pushed their Chrome extensions which inserted "Kippt" buttons and links to loads of websites with potentially interesting links. There’s even one for all hacker news links. This created new cues for people to respond to.
For Inc to succeed it needs to cleverly introduce more cues because email works fine for this particulare habit loop and will be hard to replace.