I'm convinced that what killed Google+ was their slow rollout, which was an absolutely bone-headed move that I'm really surprised a company like Google would make.

Google+ was a social network. For a social network to have any value, you need your friends to be on it as well. By making it invite-only and throttling how many people could join, they guaranteed that most of your friends weren't there and COULDN'T be there. There were so many memes being made of Google+ being described as this amazing party you got invited to, only to get there and find there's nobody there.

GMail being invite-only at first wasn't a problem because using GMail didn't require everybody you sent/received e-mails to/from to be on GMail as well.

Yes. It seems like they learned the wrong lesson from both Gmail as well as Facebook.

With Gmail, the invite-only rollout had the effect of making the service seem exclusive and valuable. I remember getting my invite from a friend and being “let into the club”. (I didn’t take it that seriously, but the feeling of being “in” was still kinda there). And as you note, email still works across providers, so no network effects were harmed.

Facebook also had early exclusivity, but it was entire cohort schools at once! If I was allowed to create a FB account, that meant all my classmates were also being admitted to the club.

G+ didn’t start with any natural cohorts, so being rolled out slowly just guaranteed that initial experiences were tumbleweeds and disappointment.

Then again, Facebook did a slow rollout - first it was just for American college students, then (if memory serves) it was rolled out internationally for colleges and universities outside the US, then finally it was available everyone.

This helped build word-of-mouth. While a social network has value, so does an exclusive nightclub with a long queue around the block to get in.

I've no idea, but I wonder if this was the thinking behind Google+. Build up a demand with artificial exclusivity, so you build buzz (maybe "buzz" is the wrong word, given Google's other dead project of that name). This approach also helps you from the technical side - easier to deal with scaling and other teething issues when you have a few users at once rather than the mad rush.

At the end though it was probably the wrong decision - people were already on Facebook so they just shrugged and forgot Google+ existed.

Excellent point. It's easy to remember google stuff used to be limited and "cool" and it was desirable to get an invite.

G+ was around the time that started to swing around and people didn't think Google was so amazing or desirable.