Steam: 100k (as per OP)
uPlay: 200k (rough estimate, because this is where all the sales are, where most of the third party sellers are, and for people who mainly play Siege, you don't need to boot up Steam.
PS4: 125k (according to this post, PS4 has about 25% more sales than Steam, I'll assume online player count roughly correlates: Link
XB1: 80k (according to the same post above, XB1 has about 20% less sales than Steam)
Total: 405k
CS:Go's last 3 month average peak was 633k, which means Siege needs 37% growth to match it.
Peaks of all the free weekends:
28k
39k - 28% increase from previous peak
41k - 5% increase from previous peak
65k - 37% increase from previous peak
73k - 11% increase from previous peak
96k - 24% increase from previous peak
Average = 21% increase during each free weekend