I was in a similar situation to yourself last year. Being active on this forum, in the shoutbox at the bottom of the forum homepage, and attending events organised by the site, has given me a great opportunity to meet people and form friendships. Compared to the beginning of last year where I visited primarily on my own, this year I've been at a park every weekend since the start of March and think I've only had 2 visits where I haven't at the very least bumped into someone I know, which has been great.
In terms of meet-ups, the events that TTSP organise are a great way of getting to know people in a relaxed environment, where both the organising team and attendees will go out of their way to welcome new people. I know I wasn't alone in being somewhat apprehensive of attending my first one, perhaps for some reasons similar to yourself, but the diverse group of people these events attract and their shared enthusiasm for having an enjoyable day, very quickly undid these concerns.
Maybe check out the shoutbox in an evening, and say hello to whoever may be chatting in there.