Twitter users by the thousands — or maybe even the hundreds! — stubbed their scrolling fingers yesterday at the news of a new default setting in the popular service. Previously, links to photos or videos in tweets hosted on Twitter servers did not appear in a user’s “timeline.” Now, visual previews “will be front and center in tweets,” the company announced.

By Web standards, the Twitter change was incremental. But as Wired‘s Mat Honan and BuzzFeed‘s John Herman explained, it nonetheless infuriated longtime users who like their information-compressed, character-based Twitter just the way it is. These veteran users regard the inclusion of visuals to their Twitter timeline like the addition of a fistful of arrowroot to their miso soup, and don’t care that the visuals will make it easier for the company — as it approaches a public offering — to sell ads and compete with the visually richer Facebook and Google+ services.

Aside from growling about it on Twitter, what can the 140-character minimalists do? Not much. Free Web service outposts like Twitter, Facebook, Google, SkyDrive, Dropbox and the rest can change their features and their terms of service (ToS) at will unless the Federal Trade Commission intercedes with a privacy audit or ruling. The only real resort for irate users is to delete their account and take their cheapskate ways to another free service. For the most part, this never happens. Back in 2010, “Quit Facebook Day” organizers convinced only 33,313 out of 400 million users to disconnect from the service, as Alex Howard reported. It turns out to be easier for someone to leave a marriage than it is to abandon a Facebook or Twitter account. If you’re fed up with your marriage, there’s a bottomless stock of potential spouses. But there is only one Twitter and one Facebook. Grow heavily invested in a free service — Google would be mine — and you’ll grudgingly surrender your golden retriever, your first-born, and your left kidney if and when the new ToS require it.

It’s only fair to offer some sort of defense of expanding terms of service: Free online services like Facebook and Google are, after all, not static products like a head of lettuce, a new car, or even a piece of packaged software. Because the product is changing almost weekly, invariably becoming “better” in the company’s eyes, ToS creep is justified. A “new” product requires a new agreement from the user.

But everyone now knows that the ToS noose is designed to grow tighter and tighter until it turns customers into the service’s revenue-producing slaves. Not to single Facebook out, but its history perfectly illustrates the progression of a free service — from partner-in-symbiosis with users, to bloodsucking parasite. An Electronic Frontier Foundation timeline and a Matt McKeon visualization, both from 2010, demonstrate the speed with which Facebook’s privacy policies eroded as the service got larger and desired more user information to sell to advertisers. Locking users in is a profitable strategy. Recruiting millions of users and selling them to advertisers has garnered Google a projected 33 percent share of the global online advertising market.