This is because some apps send/receive information from the net periodically (like do you have new mails, new updates and so...).
The things you can do is to manually enter all the apps you don't like to be running, go to options and disable all SYNCing options and unsubscribe.
Also if you don't use some app (like me f@cebook and tw3tter) you can uninstall them and never see em again :>
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