I bought Far Cry 3, off Steam, when it came out with the intention of playing online with my friends. At the time my computer was running Windows 8, and I was only ever able to get a single game to play online before it stopped working. Assuming it was related to the game not officially supporting Windows 8, I shelved the game until I could come back to it later. I recently came back to it, my computer now running Windows 8.1 with the same issues. My computer has gone through a build and has also seen a new motherboard (unrelated). The router between the computer and the internet has also been replaced. I don't really know what else to try, but I'd like to be able to play the game I bought. Ubisoft forums have been less than helpful.
Considering the question was posted in 2014, it is sad to say that the multiplayer component of the game was already dead by this point. Far Cry 3's PvP deathmatch had a lifespan measured in months.
Apart from Co-Op with friends it was very difficult to find a lobby for multiplayer aspect of the game. Even one month after the game's release people complained about low player counts. And speaking from experience I can remember waiting as long as an hour for a lobby to play on back in 2013. I can fully understand why you were not able to find anyone online in 2014.
That being said, there were technical issues with the game that prevented multiplayer from working. In some cases, basic troubleshooting could help improve your multiplayer experience. Updating the, UPlay, and limiting your computer's firewall all allegedly help find other players. I haven't seen anything related to Windows 8 specific issues and I doubt that's the reason you can't find any players.
There just isn't anybody playing this game online anymore. Hardly anyone in 2014, and definitely not anyone now.