The number one down-side to being an introvert is... being an introvert.
It doesn't really matter whether one calls it a lack of social skills, or simple shyness, I've been told I am both shy and socially stunted.
However, I'd say it's a personality trait, given that I've always had a hard time making close friends. I'm too abrasively honest with some people, and too shy with others. Both have cost me friends.
I look at photos of parties I declined, simply because it would have meant seeing people I burned bridges with, and have too much awkward history, rather than go and catch up with the people I miss and havent seen in 2 years. What the hell?!
The psychology side of my brain sits there and knows exactly what I have a hard time with, and how to go about fixing it: it should be a simple case of taking a deep breath and throwing myself in the deep end. While it may take conscious, exhausting effort at first, it will soon become second nature. BF Skinner went out and spoke to something like 20 strangers a day until he could do it naturally. With practice, he made himself more extroverted. Yet, something holds me back, like when you catch your breath on something as you exhale. And then you can't.
Mike reminds me every once and again, in his annoyingly frank tone, that if I don't make the effort I'm going to end up sad and alone. Never mind the fact that I'd stick with the guy forever if I could. But, as narked off as his tone makes me (it reminds me of my mum), he's right. Something's gotta give. And soon.