There are various other factors that determine a page's PR. You are not doing anything wrong
All your twitter backlinks are from your site because you have placed a sitewide link to your twitter account from your site.
The PR you have on your twitter page is mainly due to the high PR value of the twitter homepage, i.e. PR9.
Even if you had 0 backlinks to your twitter account i would not be surprised if your twitter page achieved a PR of 6 or 7 depending on traffic and some other factors.
So i think its not right to compare the PR of a fairly new independent site with an established top-level site.
Don't worry, you don't suck in Seo :P
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Don't worry about your PR. What really matters is the search rankings. Also note that PR is updated at Google on a daily basis. It's just that they don't update the toolbar very frequently. You site could have higher PR than you think and you wouldn't even know it. That's why it's more accurate to judge your site on traffic and search rankings.
I also suggest that you add NOFOLLOW to all the sitewide links to twitter. You are linking out and gaining nothing in return. As twitter is NOFOLLOW.
Yeah I will change twitter link to nofollow. Now I see that it is similar with all twitter accounts and websites of owners. Even with Kevin Rose (founder of Digg) his twitter acc has pr7 and his website kevinrose.com pr6.
I think Twitter is just a more trusted domain. It is used by millions and any page on their site would get a higher PR than an independent and relatively unknown domain.