this.
i think BiFests etc do count as more than "just" a social event (not that i'm dissing the existence of social events - they may not be activism but making them happen is still work and worthwhile) because creating bi space in a still largely biphobic world is important. People come out for the first time at bifests (i know at least one person did at Oxford) and that's really precious.

Providing safer sex information and supplies is activism.
Encouraging people to question mainstream relationship/lifestyle options and finding what works best for them is activism.

And besides in my more cynical moment i wouldn't put so much work into just social events - i'm not that social!