I agree that we need a bit more data, and that time will tell. For what it’s worth, here’s the data accompanied by a seven-day moving average, and with the holes filled in:
If I put a trendline on that average, it heads upwards – but that doesn’t mean anything with so little data. 
I’ll keep running this experiment for a while; I know it can’t ever give us a definitive answer, because there’s no exactly comparable control group. As @HAWK notes, the use of likes is probably also at least somewhat culturally bound. Also, as @tophee notes, the forum type also plays a role; a large part of our forum is given over to suggestions, with the voting plugin enabled, so those topics won’t have any likes on the first post.