web services can change at will - especially free ones
Jeff11 beat me to it. I love free services such as twitter and gmail (not facebook so much). But:
1) If you have not paid anything for a particular service, there isn't any contract and they have no obligations towards you. If you don't like changes they have made, you really don't have any say in the matter (individually). Collectively you may have a say, but that is up to whether the provider is worried about general public opinion and keeping non-paying users happy, or if they are decided to focus on paying customers. It doesn't matter one bit if you put a lot of effort into this service -- as a user, setting up a big wall or contact list or lots of photos or what have you; or as a developer putting in a lot of effort developing a nice app or service or meta-service or whatever. That's a lot of effort on your part but on it's own doesn't make the service provider a penny.
2) Web services really are not like desktop software; if you don't like a particular software upgrade, you can choose to keep using the older version of the software. Not so for web services (in general... of course as you move from gmail or yahoo to some php software sitting on you web server, you CAN keep using the older version.. just be careful you don't run software full of security holes!)
3) They may even go out of business, or discontinue your service! I don't see twitter or gmail going down the tubes any time soon, but I've seen several cases where someone is like 1) "That was the only copy of my website!" (after Geocities went down), 2) where someone made online backups but the backup company went tits up, 3) where someone bought rights restricted audio (from Major League Baseball!) and once MLB decided to end that experiment, they didn't even keep the rights restriction server up a single extra month, OR refund the money for the audio!