I have similar problems, but I'm not sure mine is really a social anxiety as such... it's more a deeper insecurity which probably comes from when I had clinical depression when I was 16. Unlike other posters in this thread, I actually am pretty talkative/confident with new people/in big groups/at parties, etc... basically, I don't really worry too much what people I've just met / people I'll never see again think of me, the fear/insecurity is more what they'll think of me once they get to know me more and know my personality more intimately. And it's been REALLY debilitating for me, to such an extent that I had to drop out of university in March this year after only 6 months. I'm just RIDICULOUSLY sensitive to criticism - like, last year when I first started at uni, there was a big of group of us in the block of flats I was in who always hung out together, and even though I was getting on with most people, there was one person who I thought I was picking up that he didn't like me or thought I was annoying, and just the thought of that one person not liking me was enough to drive me crazy in my head for weeks (I think subconsciously the fear was that everyone would soon think like he did). And there'd be loads of seemingly-trivial incidents which I'd totally blow out of proportion in my head... like one time just before last Christmas, a group of us were watching X Factor, and a girl who I wasn't even THAT close to just said "shut up" to me (I was talking while someone was performing I think, or something like that), and just that cut so deep that I spent the next two days isolating myself from everyone and staying alone in my room
It was only really months later when I was at counselling that I could start to see these types of things more rationally and put them in proper proportion.
Eventually, towards the end of my time there, I was becoming such an insecure mess that the relationship I was in was just totally destroyed (ironically, the fact I was so insecure and fearful about him rejecting me and constantly seeking reassurance, actually itself led to him rejecting me because he just couldn't cope). And then I spent the next few months feeling so depressed I didn't socialise AT ALL... it's only really been the last couple of months that I started gradually rebuilding my life and going out occasionally with schoolfriends, etc. I'm hoping to restart at uni next September.
To anyone else suffering from something similar, I'd definitely advise them to go to counselling asap... I know from my experience that the longer you leave it, the more bad habits you create/entrench and the harder/longer the process to undo them is. Also, don't expect immediate results from counselling either - it really took me 4-5 months of sessions to get anywhere, because we really had to go through basically my whole childhood and last couple of years in order to really understand why and where it developed it from, before we could figure out how to move forward.