Since users started complaining about Skype, Microsoft has issued official announcement regarding the problems. Apparently the company decided to end support for Macs that are running OS X 10.5.8 and below, while the OS X 10.6 Snow Leopard users are still unaffected.
Last month we reported about the possibility to end support for older versions of Skype for both Windows and Mac computers. One of the reasons for this are security issues that could affect Skype, but also its design as Microsoft wants to have the same design and features on all platforms. Here is the official message from the staff member.
there seems to be a lot of confusion around this. First, let me assure you that no-one is locking you out from using Skype on OS X 10.6 - 10.8 and there is no need to upgrade to Mavericks or (the new OS X now in dev preview) Yosemite if you don't want to.
All OS X 10.6 - 10.8 users can head over to http://www.skype.com/en/download-skype/skype-for-mac/downloading/ and get version 6.15. This is a very solid release and you can still use all of the main features without any problems (i.e. group video, group chats, screen sharing etc).
Note that you should only use the link above, it serves you with a correct update for your OS version, direct links (to DMG files) can work, but are not guaranteed to do so.
That said, we still encourage you to stay up to date, meaning upgrading to Mavericks and getting the latest Skype.
Hope this clarifies things a bit.
If you have problems updating, please let us know in this thread and I'll try to help you.