Yes. Download.com is the 173rd highest traffic site in the world. They're probably pushing over 100-200k downloads a month at least. Nothing compared to the Apple or Android app stores, but still a significant number.
Assuming an average download time of 1 minute, that's merely 5 or so simultaneous downloads at any particular point in time. That is nothing.
I'm guestimating that you're off by an order of magnitude.
According to Alexa, somewhere between 0.6% and 0.8% of the entire web goes there every day.
Download.com users are probably over-represented on Alexa simply because of all the tracking crapware they have installed.
I pulled my apps from download.com years ago because the traffic was neglibile. It's probably alright if you are in the top ten for a category, otherwise there are much better ways to promote your software.
I'll often use File Hippo, but they've always seemed "cleaner" to me somehow. I even use their updater on some computers, which I wouldn't even consider doing with those other sites.
Only for Windows...
Yes, but the Venn diagram of them and serious nmap users probably looks like a figure 8.