Digg is mostly useful because if you submit your hubs to Digg - they get indexed within an hour.
If your hub isn't indexed - that means that search engines do not yet know that it even exists.
Traffic from digg doesn't matter, and is not the reason why you submit to Digg. You submit to Digg for the purpose that Google knows your hub exists an hour after submitting to Digg.
Twitter? I hate twitter. I see no point in twitter other than that other people seem to like it for reasons that I can not understand. You also want to submit your hubs to twitter - just because.