Personally, I have built up quite a huge group of PSN friends in the past 2 years playing GT5. I've met pretty much every PSN friend I got through GT5 and 90% of my PSN friends are cruisers like me. Most of us know each other since months or years even and we keep coming back together for epic lobbies on GT5 (mostly) and other driving games (now and then).
The limit's 100 people but that fills up quickly. I've met these people through playing in GT5 lobbies with them (most of them), when someone is a generally nice driver/cruiser or appeals to me when chatting (usually both), I add them, but at the moment I add like 2 people a month or less. My list is pretty much filled up with great people and I'm satisfied with it.
Did I just write up 2 paragraphs repeating basically the same while bragging about my virtual friends? Oh dear...