after turning 31 this year, this post is pretty relevant to me!
i met my best friend through okcupid about ten years ago*, and other friends through the scene.
initially, i was flirting with him subtly and he was like "all the guys on this website want is sex", which made me back down; we had a good match score, and he's insane, but we've been great friends for ages. i met guys through okcupid for dates and sex, but they weren't really that great; i would go for the high match scores and then find that we might be too similar, or something in person that didn't come across online.
ANYWAY, there are also some friends that i've made through grindr, but i've found that putting myself out there and showing up to things (like parties) can certainly grease the wheel.
i feel like commonality is a good foundation and that someone who's consistent with their first impression will make a good friend. friendship is a collaborative effort.
lastly, i've been reaching out occasionally to people on facebook that i don't often see in person. i feel like i have a greater amount of acquaintances than close friends.