There was problems with no only joining this server but all multiplayer servers except those run on LAN support. The multiplayer sessions at Mojang malfuctioned for around 2-3 hours but joining multiplayer servers is now fixed.
Although joining can often be slow right now as mass amounts of people are trying to join the servers they so want to play, it's still possible to join.
You can go to www(dot)help(dot)mojang(dot)com and see if any of the services they offer are offline if you are ever experiencing any trouble as well. ^-^