Data is available in open formats for easy download. The best sign that a website or service has the preservation of its users' data in mind is the ability for users to make a backup copy of their data through the service itself. If there's no back-end tool for downloading copies of your content, you may be forced to scrape the data manually, so anything that saves you the trouble of having to do so is worth noting. The wiki-creation site Wikia.com, for instance, lets you save whole wikis or individual pages into plain text files either for archiving or offline editing.
Interestingly, Google has been making major strides in this area. When it recently started beta-testing its Google+ social network, it added extensions to allow personal data (contacts, circles, etc.) to be exported via Google Checkout. The real test of such a feature, though, is how useful it'll be to transport your data into other services.
Data tools are provided by the service or third parties. If you don't have direct access to your data through the service's own Web interface, the next best thing is an application that can pull that data for you via one of the service's APIs. You might have to do some programming on your own to take advantage of those APIs, but it's a good idea to look around first -- someone else out there might well have done that work for you and made the results freely available.
Andrew Reichman, principal analyst at Forrester Research, says any service you use should be considered proprietary, even if the provider of the service advertises its own exit strategy. In other words, take any claims about data portability with a hefty chunk of salt. "Even with standards [for data interchange], you are still at the mercy of the administrators and policies of the company operating the equipment on your behalf."
Terms of service. The ToS for almost any service these days is worded to within an inch of its life, with almost every conceivable aspect of the service's functionality covered. "Paying close attention to the SLAs [service-level agreements], contracts and penalty structure related to non-performance of SLAs is critical," says Reichman. "Having an exit strategy, or at least some discussion about what would happen in the event the customer wants to pull out or the vendor cancels service, is an important preliminary step to take, prior to committing to a given vendor." The fewer details about such things in the ToS, the more wary you should be.
George Hamilton, an analyst at Yankee Group, is even more insistent on this point. "Caveat emptor," he says. "Know how the service provider protects stored data and data in motion, and how it is backed up."