Fidelius charm and Tabooing the name

[Wendy]

No: HPFGUIDX 173981

I have a question about how exactly the Fidelius Charm works. I think I understand the basics but I wonder how powerful it is. The trio are found on Tottenhan Court Road because they say Voldemort's name. In number 12 they use his name twice and the Fidelius Charm isn't broken. There seem to be Death Eaters that arrive a day or so later that sit outside and watch, but no one is able to see Grimmauld Place. So is the taboo not strong enough to break the charm or does the Fidelius Charm work that well? 

I don't know what other kinds of spells people are using to keep themselves safe but they seemed like they broke quite easily when Voldemort's name was used. 
   
Wendy