In the past few days, Ello has seen an incredible influx of new users. Its network more than quadrupled in size and there's no sign of it slowing down. As I'm writing this, I'm told that Ello has gained nearly 20,000 new users in just the past hour. A quick scan of my Twitter and Facebook feeds reveals a whole slew of my friends have joined the service, which is a remarkable achievement considering Ello is still at the invite-only stage. Only existing users can invite those looking to sign up, which means it's mostly spreading by word-of-mouth. Ello had to stop taking invites because its servers were slammed. People are even selling Ello invitations on eBay for $100 a pop. So what's the big deal with Ello? And why is it so popular all of a sudden? The short answer: It's not Facebook.