Every time you type one of those urls, some domain parking twit gets another link to their site and who knows how many more hits.
The Internet Assigned Numbers Authority (IANA) has reserved an url or two specificly for examples.
If you use Example.com, Example.net, Example.org, or Example.info you are not going to be helping deliver traffic to someone you don’t know anything about. These four Example.TLD (top level domain) addresses are IANA_RESERVED and you can use them safely.
Note that other example sites like .US .be .dk .de .co.uk etc are all owned by different companies. Only use the four mentioned above.