By spring of next year, you should be able to log on to Yahoo Messenger with Voice or MSN Messenger 7 and chat with users of either program. Yahoo and Microsoft announced this morning that they'll make their IM services interoperable, so users can even share emoticons and contacts. Instant-messaging apps hasten to outdo each other with features such as VoIP Web-based calling , video chatting, and blogging, but most still don't talk to each other. Joining forces, the two instant-messaging services could share millions more users than the king of instant messaging, AIM. In July, AOL's instant messenger counted 30 million users, while Yahoo and Microsoft's IM services each had about 25 million, according to ComScore. The Yahoo-Microsoft partnership leaves the fledgling Google Talk in the dust.