Skype vs Yahoo Messenger

Skype is a software application that allows users to make voice and video calls and chats over the Internet. Calls to other users within the Skype service are free, while calls to both traditional landline telephones and mobile phones can be made for a fee using a debit-based user account system. Skype has also become popular for its additional features which include instant messaging, file transfer, and videoconferencing. Skype alternative for enterprise is called Skype for Business. Secure Skype alternatives are Signal and Telegram.
Yahoo Messenger
Yahoo! Messenger is an instant messaging client and associated protocol provided by Yahoo!. Yahoo! Messenger offers PC-PC, PC-Phone and Phone-to-PC service, file transfers, webcam hosting, text messaging service, and chat rooms in various categories.
Face to face in the news:

2014 - Skype for now available worldwide to catch up with Yahoo Messenger

The opportunity to make video calls via Skype right from your browser is now available for all users (previously this feature was only available in US and UK). In fact - this is Skype's web version, however, it doesn't mean that you don't need to install software on your computer. You still need to install browser plugin. But the plus is that you don't need to create a separate Skype-account. You just need the universal Microsoft account. Besides the global expansion, now can make video-calls in HD. Thus, Microsoft's is playing catch up with Google's GMail, that already has video calling feature via Google Hangouts. However, the advantage of Google's service is that it supports free group video calls for up to 10 people.

2011 - Skype acquired group-messaging service GroupMe. Beware Yahoo Messenger

Skype GroupMe
Mobile group-messaging services hit the social Web this year. Basically they are designed for communication and interaction with friends and family, but (as we know) such social technologies are rapidly find their way into Enterprise. One of such services GroupMe was yesterday bought by Skype for as much as $80 million. GroupMe allows to create a group and send text messages, photos to its members, start conference calls and share your location. For example, you send a message and all group members instantly receive it. Or you can call all them at once. Such a service could be useful for mobile work-groups, which need real-time communications. For example, equipment service companies.

GroupMe works on smartphones (iPhone, Android, Blackberry, WP7) via the Internet connection, or on any phone via SMS. True, if the message is sent via SMS, you will pay for this to your mobile operator. And you will pay for sending SMS message to the US number. Conference calling is available only in the U.S. too. The max number of users in one group - 25 people. Until now, the service was provided free of charge.