What are the chances of winning a giveaway?

It's simple maths and depends on the giveaway entries. If you want to see the % for each GV use the Steamgift addons.

Win Chance as percentage: (amount of participants / 1) * 100
This was fairly obvious though.

dunno, lower than zero?

I've been lucky, I just won 3 giveaway in 3 days. 3 litle game, but a month ago or so, I won a big game on steam companion. I have no complaint.

won several games when I came here last year. This year, compared to my number of entries, I haven't been so lucky. But it's still fun to try :) There's a lot more people, which probably means a lot more games... but also a lot more entries.
But I've learned about games I want, which I wouldn't have noticed otherwise and I've gotten games through free promos, which I wouldn't have known about if I hadn't seen the forums here. So that's a good thing. :)

Entered 1000 Giveaways. Won 0 :(

Apparently my probability is currently 15350.0%

do they no longer teach basic maths as school anymore?

50%, either you win or you dont.

I win 1 game in fake giveaway :) so i feel luucky.. :)

Huehuehue.

The first giveaway I won was a big developer one, I got a beta key for Tryst.
Second one was Cave Story+

I seem to win once a year.

We used to have this neat 'expected wins' feature telling you just that, over all your raffles. I still miss it :(