Google Plus was a last ditch effort by a company to get, and keep, any kind of foothold in a market that they continually failed at. It was to my knowledge, at the very least, their fourth attempt (Orkut, open social, Google Buzz, Google Plus... am I missing one?). Calling it a "very real competitor" seems a bit overstated.
Like my comment below states, Google Plus grew to 30M users in their first month and 90M by the end of the year. During this time they were considered a real competitor and many people were predicting that they could dethrone Facebook and that there is nothing that says another competitor won't come along and do to Facebook what they did to Myspace.
Sure, looking back now it's easy to say Google Plus was a failure, but at the time people were really excited about it. Check out this graph of their growth:
And again, the reason for my post is that I wanted to point out there were people worried about competitors taking away market share from Facebook during their IPO just like there are people worried about competitors to Snapchat. It's only now that it's easy to discount those competitors because we see how it played out, at the time people were really excited about it, check out this graph: http://i.imgur.com/yQU2Y2z.jpg
There were tons of skeptics of Google+ from the get go. Their way of getting users was dubious at best. We don't know how many users were ever true active users vs people who signed up because of having a Google/Gmail account already and checking out Google+ once or twice. Or creating a Google+ account and never coming back or coming back accidentally. Those all still counted as active users. It isn't just hindsight. People including me were calling this from day one.
IG has grown around 45% in the past year. Snapchat has grown half that, close to 23%. IG is the more established social network and growing faster. That's the worry. Sheer numbers make it look so much worse. Snapchat added around 50M new MAUs over the past year. IG did over 4x that at over 200M.
I remember seeing charts similar to the one you posted and dismissing it as BS back then too. As did others. It's a vanity metric. Without knowing the engagement, it is mostly meaningless. And most signs pointed to there being weak actual engagement. Especially if you look past those user's first one or two enhancements of G+. Most prob never engaged again. The same can't be said for the others. FB's first 50M users were by and large engaged. All the others were too, to different degrees. All far above Google+'s.
Google Reader and YouTube both predate G+, and were specialized social networks. One for links of any kind (you could share anything), and the other for videos.
