I get the criticisms against Instagram. There's a lot of validity there. But seriously, at the end of the day, if you don't like the changes...stop using the service. There's no entitlement here.
Although the entitlement mentality shown by users is certainly an overreaction, primarily by users who don't understand the corporate structure that they've bought into by their use of the services, I can't say that xkcd comic you quoted in support of your position is valid.
These companies turn the data we the users produce from our individual actions and queries into a marketable asset that they can (if everything goes according to plan) profit from. This means that their service is not offered altruistically or even unwillingly; their persistant existence relies on our using their services.
I don't want to come off like a privacy nut, because first of all it would be hypocritical given that I am a user of Google and Gmail, but something just doesn't smell right to me about this situation we have all gotten ourselves into by using these products.
And that the service that you sign up for is probably not the service that you'll see a few years down the line when the pressure to monetize is up. Until then you shouldn't be asking if there is a catch, but what the catch will be.
Period.
What you need to understand is this: IP has real value. Period. So when a company secures limited use of yours in return for a specific service under a specific set of terms, they have no business retroactively changing those terms, any more than they would in the case of a cash transaction. And stating that the terms can be changed simply means the company is free to evolve the service, and renegotiate accordingly. It doesn't mean that they can secure your agreement under one set of terms, change everything, and retain unbroken consent.
Every contract is based on an exchange of value. Both parties provide something, and both parties take something away. While it's fair to classify as "customers" those who pay cash, it's a mistake to think that other parties to the transaction are less than fully protected by the laws that govern exchange in general.