Would you pay for Foursquare or Twitter?
wadehammes.com
wadehammes.com
I'm sympathetic to the (probably true) view that they never would have grown to their size (and usefulness) if they were paid services, but that doesn't mean they can't just flip the switch today.
As someone who runs a freemium site, I know how important real (non-ad) revenue is. I also don't mind paying for good quality service or content. Finally, paid services have fewer scammers and bots.
Twitter — possibly. It depends if twitter had the same pervasiveness as it does today. In reality, that's probably impossible: twitter's ubiquity is due in large part to the lower barrier of entrance. So it seems that paying for it reduces the utility of the product and, therefore, makes it not worth the money.
In summary: no.
Foursquare: Nope. It seems to me that the core of Foursquare is a feature that can be replicated on any site, and the only thing that makes it unique is the badges and the potential deals with retailers. Neither of which I'm willing to pay for.
If I don't care about the service then of course I won't.
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So imagine if you had only 2% of your friends on FourSquare, or Facebook, or Twitter. Much less valuable, no?
Also, These companies can make far more money via other business models.
Users > Revenues for some consumer startups. Grow now, monetize later.