Price and cost matter a great deal here. Although the price of G+ was zero, the cost quite high - the user's time. G+ was hard to understand for most people. Google tried to give it a language of it's own that took people effort to learn - groups were "Circles", chat was "Hangout", and so on. It didn't instantly click in the way that Facebook or Twitter does for most people. Given that, the value proposition for G+ wasn't good enough - the benefit from actually using it returned less value than the cost of the time to spend learning how. That's the number one thing for free products - the cost is never zero. Everything costs something, just not necessarily money.

The image of the product was equally bad. Google rarely gave a clear picture of what G+ was actually for - it looked a lot like a social network, but Google's messaging was always "It's not a social network! We're not competing with Facebook!" They always told people what they weren't, rarely what they were. Frankly, I still don't know. That's ignoring the forced "G+ is your Google account" thing because many Google products could actually be used without G+; G+ was never your Google account.

As for critical mass, if you haven't got that when you have 500 million users, you're obviously getting something very wrong.

tl;dr G+ didn't provide enough value to users to make it worthwhile spending time with the product, ergo it was a bad product that people didn't spend time with.

> the cost quite high - the user's time

That's a good point. I still don't really "get" Google+, and I find it somewhat counterintuitive.

> tl;dr G+ didn't provide enough value to users to make it worthwhile spending time with the product, ergo it was a bad product that people didn't spend time with.

I was challenging your "few users ==> bad product" line of reasoning in general, not necessarily the statement "Google+ is a bad product". Sorry, I should have been more clear on this.

Still, I stick to my point that there are more factors influencing the market share of a product than its pure quality. Even if I made a better Facebook clone, I wouldn't gain any noteworthy market share.

That's a fair point. I really just drawing a distinction between accounts and users. G+ had hundreds of millions of accounts but relatively few users, certainly a single digit percentage of total accounts. That's a strong indicator that people aren't coming back after their initial experience or that they're signing up to something they don't want in the first place. Neither is a sign of a good product.