Scott McNealy once noted, "You have zero privacy anyway. Get over it." Maybe not so much "get over it" as plan accordingly.
E-mail is a slightly different matter. The law makes a distinction between intercepting e-mail in transit and obtaining stored e-mail from a service provider's servers. The distinction made sense in the 1980s and early 1990s when downloaded e-mail often sat only on the user's computer. If the government wanted the records, it had to go to the e-mail recipient.
These days, most e-mail is held and stored by third parties. So the government claims the authority to read someone's most intimate communications, including stored chat sessions, by serving a subpoena -- no probable cause required. A person may never even know that this has been done, as there is no legal requirement for an Internet service provider to provide notice. In most cases where the government subpoenas the e-mail, it demands that the third party keep that fact confidential, at least for a while.