Ahh, I wasn't aware of that.
Surprised they're giving such a great game like this away for free - Awesome!
And what do they mean by 'activating' it? Just playing it?
They mean putting it into your Games list by 'purchasing' it through the store. You don't even have to install it.
Well you can only assume that half the planet already has Portal and the other half weren't aware it existed and the only way to not know it existed is to not know of Steam
So really its a fantastic marketing technique, an old game that is widely regarded as 1 of the best games in recent years for free to get more people onto Steam by word of mouth and of course the subtle hints of Portal 2 in the MAC advert, also 1 of the best adverts in recent years