Hello and welcome.
Every one should use a signature line. whats good about a sig line is you are promoting you and your business.
Sig lines go at the bottom of your e-mail, blogs, post, comments, and when every you do any writing.
a signature can look lite this:
A signature line only works if you send a lot of e-mails. The best place to use sig lines is in forums where you post frequently. But I'm one of those people who does more reading than posting. When I post in forums I try to make my posts more than just a couple of lines.
Also with e-mail signatures you want to keep the length of the e-mail to a minimum because if it's too long people get bored and delete it without getting far enough to
see the signature. Signatures are ok but they're really not that effective considering the
amount of e-mail we get as opposed to the amount of e-mail we send.
I agree a signature line can be a useful tool. Like every tool you must use the right tool for the job. Please visit Social Networking Basics & Etiquette (item 7) for some friendly advise on how to use a signature line in a social networking environments.
I'm not sure it's always about traffic. Signature lines can lead to traffic or it can also lead to back links if the site is a dofollow site. The back links are more important than the traffic IMO because they will remain on the site and lead to better position in the search results.