[Age of Ignorance]

I would say that it's a great opportunity to meet people, but I'm not sure how effective it is. Speaking anecdotally, I know a few people who have lasting relationships that have begun through online dating sites. I'm assuming that's because most people go into these "dates" knowing exactly what the other wants, and they don't have to fluff around with all the bullshit courting phase. So, personally, I'd say it's effective, but it's really hit and miss.

As to how you'd formulate a message, I'd just introduce myself first. Tell her/him what your interests are and ask if they share any of those interests. If they respond in the positive, and they seem keen, ask if they'd like to meet up for a drink or for dinner or whatever it is you're planning. If it fails, thank them and move on, I guess.