I have a small, niche website business so I know a little about this (we aren't on the scale of these guys but do low 7-figures).
Wix and Squarespace at least are spending a ton on PPC customer acquisition. One analysis we saw put their AdWords spend at $10-25K per day (though, I'm not sure how accurate that is).
Also, both Wix and Squarespace advertise on terrestrial radio and television. Squarespace had a Super Bowl ad and I regularly see their ads on Shark Tank (ABC). I hear Wix ads quite often on AM radio.
Squarespace was on Inc's 5000 list in 2011 with 2010 revenues of 11.7M.
Wix went public recently. They had 2012 revenues of $43.68M with a loss of $15M [1].
In the "general purpose business website" space, I think all 3 (Wix, SS, Weebly) see it as a "winner take all / own the market" play (it probably is). So, they are all spending as much as possible to acquire customers.
Wix clearly isn't profitable. I doubt SqaureSpace is. I think it's admirable that Weebly is ... but I almost wonder if they need to spend more to compete.