Firefox is a great alternative to a sucky dominant player (IE). Inkscape is a great alternative to expensive dominant players (graphic software). The problem with this case is that Skype is excellent, free and dominant. There is no incentive for users to switch, other than being open source (which appeals to people like you and me, but the masses don't care)
...unless you're on Linux.
It's Galaxy S Android client doesn't work and the problem hasn't been addressed in many months.
I'll be one of the first to jump ship.
Which cell phone manufacturer has "taken all"?
You seem to be basing your statement on the impossibility of interoperability, which is an odd position to take, at least if you actually think about it.
Phone numbers are universal. It's not like an iPhone user on AT&T can only call other AT&T users[1], or that T-Mobile customers can only call other T-Mobile customers. Skype limits me to reaching out to other Skype users. FaceTime only works with people who have bought into the Apple ecosystem.
[1] Although one might suggest that these iPhone users can, in fact, call no one.
1/ policy makers do not like running a third-party closed-source obfuscated binary with undefined behavior aggressively evading firewalls inside their business or country (in the case of China).
2/ the network effects are not nearly as strong as, for instance, ebay's. i do not care about being able to call random strangers on skype, only about my contacts. the scope of the network effect is to some extent that of a social group, not an entire market.
But more importantly Skype can fix the ux. They cannot fix the freedom issue (and if they somehow manage to do so anyway, great).
So Alice has to install Google talk to call you, skype to call Bob and SIP to talk to Carol
Meanwhile Bob has microsoft phone to talk to ....
Yes, but do you remember surfing the web in the days between Netscape 4 and Mozilla being launched, and quite how horrible it was for anyone with a technical mind who just wanted to be able to, e.g., debug some Javascript in a timely manner?
If FF was only available for Linux it would never have taken off. Attempting to challenge an already hugely successful product that's works excellently on Windows is every bit as pointless as writing yet another office suite.
The real tragedy is the engineering effort that will go into this, when it could be spent on parts of the Linux system that really matter to the average user, e.g. sorting out the mess that is sound.
See firefox, inkscape as successful examples.