For personal communication, it will depend on what your friends use for VoIP communication as to whether you'll need Skype for Mac or not. Skype for Mac is almost a prerequisite for anyone serious about business communication. ViberĀ is one alternative that is cross-platform supporting on desktop and mobile environments and has video calling capabilities. In some countries, other chat applications have become more popular than Skype or are the preferred choice.
Local numbers can also be called with a package of calling credit bought from Skype. Make voice with a microphone or video calls using a video cam and microphone connected to the Mac. Send mobile text SMS messages and instant messages over the internet. Cross-platform, Skype currently has 500 million registered users and over 25 million active users at any given time.
Skype for Mac is one of the most popular instant messaging and audio / video chat apps for the Mac OS X system. " A fully-featured Internet Telephoning soft."