The UK is a founding member of Nato, and when Argentina invaded the Falklands it was a direct attack on British territory. Why wasn't Article 5 invoked?
For reference, Article 5 of the North Atlantic Treaty is
The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area. Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security.
Yes, I know the Falklands technically is not in the North Atlantic, but neither is California and I'm pretty sure that if 9/11 had happened in San Francisco instead of New York that the US government would've invoked Article 5 just the same.
Add on to this question, if one of the Overseas departments of France like Guiana was invaded - would they be able to get Nato involved?
The Article 6 of NATO charter defines the geographical area covered by collective self-defence clause of Article 5. It specifically excludes areas south of the Tropic of Cancer (latitude about 23°N), so everything in the Southern Hemisphere is automatically out of bounds for NATO.
As you can see, the charter originally included the "Algerian departments" of France, which were considered to be part of France when the charter was drafted in 1949. Other colonies are for all intents and purposes excluded by the wording; this was done deliberately as many NATO members (United States included) had very little wish to be drawn to post-Second World War colonial conflicts waged by the retreating empires. (And how well that turned out... but that's a different discussion.)
After Algeria gained independence, NATO's North Atlantic Council decreed that the relevant clause became inapplicable. Similarly, when Greece and Turkey became members, the original definition was revised; but at no point did the alliance cover the former colonies, Algeria obviously excepted.
Full text of the original Article 6:
For the purpose of Article 5, an armed attack on one or more of the Parties is deemed to include an armed attack:
on the territory of any of the Parties in Europe or North America, on the Algerian Departments of France, on the territory of or on the Islands under the jurisdiction of any of the Parties in the North Atlantic area north of the Tropic of Cancer;
on the forces, vessels, or aircraft of any of the Parties, when in or over these territories or any other area in Europe in which occupation forces of any of the Parties were stationed on the date when the Treaty entered into force or the Mediterranean Sea or the North Atlantic area north of the Tropic of Cancer.
(Source: http://www.nato.int/cps/en/natolive/official_texts_17120.htm)