Here's how:

1. First, click the person's name in your Messenger list once to select the contact. Then click the Actions menu, and select Send My Contact Details.
• To send your info during an IM conversation, just click the Actions menu in the conversation window, and click Send My Contact Details.
• To send your info to someone who's NOT in your Messenger List, click the Actions menu and click Send My Contact Details. Click the Other Contacts tab and enter the person's Yahoo! ID, Windows Live™ ID, or email address.
2. After you pick a person to receive your contact details, click Next to open the Contact Details dialog box.
• You can edit or add to your contact details before sending. Also, you can choose which contact details you wish to send by clicking the checkbox for each detail...or just send all of it by clicking Select All. Click Save & Close when you're done.
3. Click Finish to send!
• If you've entered an IM ID, your friend gets an instant message saying that you've sent your contact details.