I strongly recommend you to not use Skype but open, standardised protocols like Jabber for chatting and SIP or the Jabber extension Jingle for (video) phoning. This is why:

• Skype is not Open-Source but closed source software.
• Skype ltd does not want Skype to become interoperable with other protocols or software. Hence it uses a secret protocol and tries hard to resist reverse engineering.
• Skype is only secure by its obscurity. Some audits have been made with mixed results.
• Skype behaves like spyware: Some versions snooped the local system for serial numbers of Motherboard and BIOS.
• Skype ltd is owned by eBay, a $17 billion multinational which some consider playing unfair games with their customers.
• Skype ltd helps undemocratic regimes to censor messaging:
  Judge yourself what president Josh Silverman's statement reveals about Skype ltd's ethics:
  • It is common knowledge that censorship does exist in China and that the Chinese government has been monitoring communications in and out of the country for many years. [...] TOM [the chinese company running Skype IM in China], like every other communications service provider operating in China, has an obligation to be compliant if they are to be able to operate in China at all.
• Skype ltd can eavesdrop calls:
  It is technically feasable for Skype ltd to wiretap calls (they hold the master signing key so they could sign fake user keys; and the client is closed source so it might contain a wiretap interface). In this ZDnet interview (german version), Skype ltd's Chief Security Officer Kurt Sauer tries hard to avoid that question and finally answeres
  • What we say to that is that we provide a safe communications experience. I'm not going to tell you that we can or can't listen in to that.
• Skype ltd probably provides US agencies with wiretaping interfaces:
  They always denied that they would assist government agencies to wiretap Skype calls. But if so, eBay/Skype ltd would be the only large US communications company successfully resisting the FCC, the FBI and the Communications Assistance for Law Enforcement Act, the Department of Homeland Security, the NSA and the Foreign Intelligence Surveillance Act. I consider that being very unlikely. Remember this? and this?

• Skype abuses the internet connection of idle Skype users for relaying calls without notifying them:
  When a Skype client runs on a public IP address, it is likely to get promoted to a supernode. A supernode relays calls between other, masqueraded Skype users who could not call each other otherwise. Hence a supernode provides bandwidth to calls he does not participate in itself. Besides interfering your network experience being a supernode might get surprisingly expensive when you are on a volume tariff.
  Skype ltd is very quite about this opt-out 'feature': There is small print in the EULA (3.3: Utilization of Your Computer) which allows Skype ltd to use the user's internet connection for unrelated calls. Skype clients do neither signal when they are in supernode mode nor when they relay calls. Only Windows Skype clients 3.0 or later can disable the supernode feature, and only by editing the Windows registry!
  Usually peer-to-peer networks run into all sorts of NAT problems which can only be overcome by investing into a proxy infrastructure. By making the customers to proxies, Skype can provide a reliable service without making this investment.

• Skype's Linux release lags behind Windows and MacOS versions (currently Linux=2.0, MacOS=2.7 and Windows=3.8), lacking features like HD video, SMS sending or invisible calling.
• Skype does not play nicely with modern Linux distributions, particularly with Linux' sound system (ALSA/pulseaudio).
• Skype is a network admin's nightmare:
  Skype uses a multi-port protocol with port negotiation (like ftp, port mapper, exchange, ..). Since the port negotiation protocol is not public, Skype connections are hard to control by firewalls or other means to enforce network wide security policies. In Skype 3 there are now means for network administrators to control Skype networking - but there is no Skype 3 for MacOS or Linux.

I could go on, but you get the picture. Don't use it.