> But besides that one interest, how do you go farther?
It really comes down to taking the initiative. The goal when you first meet someone you like (aside from the rare person you meet on a plane and connect with and share life stories) could be (sometimes you might not meet them again) to meet them again. Don't try to "date" them (friends or potential lovers) on the first meeting - the whole goal is to get the contact information to meet again, somewhere else, ideally doing something the other person likes. Just find out what the other person likes to do and invite them to go do it - 'I love art - and watching people enjoy art - I'd like to meet up and go with you to the X exhibit and the museum' - (the key word here is 'I'd like' phrasing - not 'Do you want to?' - express your preference, take the lead...)
I've met a fair number of the people who matter in my life through taking the initiative and talking to strangers. But, to be clear, it was not a first degree contact - it was meeting them through the person I initially met.
The problem with Google+ Circles, is that it does not allow overlap. In real life, the interesting connections occur when you overlap circles (e.g. throw a party and your improv friends meet your Salsa dancing friends). Throwing a party is one of the most effective ways to improve your social life. Indeed, taking the lead on anything is powerful. Going to an event solo may make you feel like a pariah but going with two other people - you're golden.
The ideal is the self-perpetuating party - e.g. you throw enough regular parties that you get invited to outings constantly. Doesn't have to be a Saturday night blow-out, something like a Tuesday night potluck for three or four friends, once or twice a month, will really grow your social circles.
And, most importantly, to end my soapbox (thanks for asking), just be yourself. If you are quiet and shy, don't try to be Tom Cruise 'Show me the money'. Be yourself. As much as I wanted to be the life of the party, I'm relatively happy knowing a lot of people from various worlds and circles. When I hang out with people, its usually not very regularly and usually with their good friends (and I'm the odd one out).
But the universe rewards bold action.