I think it can definitely help increase traffic to your hubs. Whenever I publish a new hub I always post it on Facebook and Twitter. Besides people you are connected to seeing the links to your hubs, their friends/followers will be able to see them too. It is free to have those accounts and you have nothing to lose by creating them and posting the links to your hubs there.
Now I forget exactly where on hubpages you can see it but you can see traffic sources - that is where the traffic to your hubs comes from. So when you create your Twitter and Facebook accounts and post your hub links there if anyone goes to your hubs you will be able to see that under the traffic sources. So, you will know if any traffic is coming from there or not.
By the way, besides stumbleupon, reddit, and digg there are other social bookmarking sites you can post your hub links on. One of them is delicious.com
There are many others but I don't remember then off the back. I'd have to google them up...