Integrate with Twitter
Specify a Twitter handle (which need not be one you own) and optionally a hashtag. Every time a tweet is tweeted from the given Twitter handle with the given hashtag, add 1 to your Beeminder graph (with the tweet text as the Beeminder comment).
Since a lot of services have auto-tweet functionality, you can then trivially beemind any of them.
Why not a custom character string, also?
I don't know if there's still the practice to sign a tweet with "^" in accounts managed by many people, but that's one of the usages I'd put in action