Microsoft seems to be using Linux for Skype master nodes
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home ~ # netstat -a -n -p|grep ESTABLISHED|grep skype
tcp        0      0 192.168.1.2:38619       78.141.179.16:12350     ESTABLISHED 1660/skype
tcp        0      0 192.168.1.2:51929       157.56.192.176:443      ESTABLISHED 1660/skype
tcp        0      0 192.168.1.2:42274       65.55.223.30:40015      ESTABLISHED 1660/skype

home ~ # whois 65.55.223.30

NetRange:       65.52.0.0 - 65.55.255.255
CIDR:           65.52.0.0/14
OriginAS:
NetName:        MICROSOFT-1BLK
NetHandle:      NET-65-52-0-0-1
Parent:         NET-65-0-0-0-0
NetType:        Direct Assignment
RegDate:        2001-02-14
Updated:        2012-03-20
Ref:            http://whois.arin.net/rest/net/NET-65-52-0-0-1


home ~ # nmap -O 65.55.223.30

Starting Nmap 5.51 ( http://nmap.org ) at 2013-03-31 22:10 MSK
Nmap scan report for 65.55.223.30
Host is up (0.16s latency).
Not shown: 996 filtered ports
PORT      STATE  SERVICE
80/tcp    open   http
443/tcp   open   https
40193/tcp closed unknown
40911/tcp closed unknown
Device type: general purpose|WAP
Running (JUST GUESSING): Linux 2.6.X|2.4.X (92%), Netgear embedded (87%), Linksys Linux 2.4.X (85%)
Aggressive OS guesses: Linux 2.6.23 - 2.6.33 (92%), Linux 2.6.22 (87%), Netgear DG834G WAP (87%), Linux 2.6.31 (87%), Linux 2.6.31 - 2.6.32 (86%), Linux 2.6.32 (86%), Linux 2.6.35 (86%), Linux 2.6.17 - 2.6.35 (85%), OpenWrt White Russian 0.9 (Linux 2.4.30) (85%), OpenWrt 0.9 - 7.09 (Linux 2.4.30 - 2.4.34) (85%)
No exact OS matches for host (test conditions non-ideal).

OS detection performed. Please report any incorrect results at http://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 15.18 seconds