Twitter is valued to be around $10 billion if its shares were to be openly traded on the market. Facebook is valued at $60 billion but at one time was closer to $100 billion and as low as less than $40 billion.
Google is worth close to $300 million and probably will go higher soon. Suppose if Google+ was a separate company, how much would it be worth?
Personally, I don't get Wall Street that can punish a company like Apple for billions but just short of a few bucks less than irrational Wall Street analysts expect while companies like Amazon and Facebook, barely managing to scrape by, gets rewarded for subpar performances.
Whatever valuation one might put on Google+, I think one metric that needs to be taken into account is the social staying power that can keep the interests of its users. There's a lot of stories out there that Facebook is losing its younger users as they move onto other social apps and networks.
The same can be said of Twitter and even Google+ with Google+ following Facebook's social structure more closely. I bring this up because of Myspace and Friendster, two social networks that were second to none but today are mere shadows of their former glorious past.
Furthermore, Facebook's better days are likely behind it and entering an era where it struggles to tread rough waters ahead. Case in point is how quick interest in its Facebook Home has dropped while sales of HTC First, the Android device with Facebook Home as its default skin, has disappointed to the point that the $99 phone can now be bought for a buck.
If anything, Google+ and Twitter has the social structure and audience that could sustain it. Twitter's ability to spread information is something that users, companies, and advertisers like while Google+, currently without ads, has a dedicated following of subgroups that use Google+ for information.
All these social networks and search engines can talk about metrics like click-throughs or impressions, at the end of the day, if your users are not engaged as they once were, it doesn't matter what clever ways you come up with to show ads.
So far, Twitter has been holding its own with Google+ closing close to Facebook in numbers while evidence of Facebook fatigue is more apparent every day.