Sure, the big moments of serendipity for me are the ones tied to geographic proximity. When a friend says they're hitting the Shake Shack ( http://shakeshack.com ) in 15 minutes, I can show up and join them, or vice versa. I don't want to email or IM all my friends everytime I'm going somewhere, but if I Twitter the message, I know I'm spreading the word without spamming people.