I definitely receive the most amount of traffic from StumbleUpon - especially if I've had a recently submitted article. But despite this, StumbleUpon is always in my top 5 list of referrals (it's currently the 2nd most popular way that people get to my site/blog - with direct input being #1, so technically it's my biggest source of referral traffic, at least at the moment.)
Twitter is the 2nd highest social network - it's in my top 10 referral list at the moment
It seems a lot of people came here by following Velveteen Mind. While missed her twitter post about it, she must have told me telepathically, which led me to google southern mom bloggers and then I found this site.
I hide my head in shame to say I just joined twitter last week, so I'll hold hope you gals are right!