Users who ignore ToS provisions about third-party applications do so at their own risk. "Legally, you could be breaking a term of service or violating copyright laws," notes Hamilton. "Or, if a Web scraper is constantly scraping a site, they could impose performance issues or become the equivalent of a denial-of-service attack."
Reuse of your content. Some sites will have a ToS provision that allows whatever you post to your account to be redisplayed in other contexts. If you see this clause, don't panic, but do read it closely. This clause typically exists for the sake of allowing whatever you post to be shown in promotional material, rotated on the site's home page or just manipulated internally.
Google's ToS, for instance, has this in paragraph 11.1: "By submitting, posting or displaying the content you give Google a perpetual, irrevocable, worldwide, royalty-free, and non-exclusive license to reproduce, adapt, modify, translate, publish, publicly perform, publicly display and distribute any Content which you submit, post or display on or through, the Services. This license is for the sole purpose of enabling Google to display, distribute and promote the Services and may be revoked for certain Services as defined in the Additional Terms of those Services." Many other services retain a similar clause.
As-Is/As-Available. This is another catchall clause that, in effect, means the service has no particular obligation to provide continuous uptime, to protect your data's integrity or even to keep the service active. Note that As-Is clauses may be a bit buried and not broken out into their own section; search on the keywords "As-Is" or "warranty" to find them.
At-will termination. Finally, some terms of service have a clause that states they can pull the plug on your account, just because. Don't be surprised if you see something like this -- it's usually in there as a catchall way to kick people off if they flaunt the rules or consume a disproportionate amount of the service's resources. You may not need to worry about this most of the time, but it may be used to justify booting you off if, for instance, you use an unorthodox or unapproved method to retrieve or mirror your data. Google has this clause in paragraph 4.3 of its ToS; Yahoo's ToS has it in section 15. In both cases, it's worded in an open-ended enough fashion to make it possible for an account with either service to be closed for no apparent reason at all.
Create an exit strategy
If you don't have major qualms about a service you're with but you still want to create an exit strategy, a few basic points are worth keeping in mind.
Keep local copies of everything that's crucial. The only storage you can completely trust is the storage you physically own, so always make sure there's a local copy of everything important. If you've already been trusting your only copies to a site, break the habit now. Any Web service should be thought of as a replicator, not a repository.
For instance, don't ever trust a remote service to your only copy of a given photo, since the service's rule about data preservation might not be in your best interest. Flickr, one of the most popular photo hosting services, doesn't allow you access to the original copy of an uploaded photo unless you have a paid account. A utility like Flump or FlickrEdit can help you extract pictures from your stream, although they will probably not be able to rescue images that aren't publicly accessible. (Flump, in particular, requires a Pro-level Flickr account to be useful.)