Here's a fun comparison from Optimal, a social advertising and analytics startup: If you look at big brands on social networks, their following seems to be growing more quickly on Twitter than on Facebook. Optimal says it looked at the data from 4,330 brands, representing a total of 3.49 billion Facebook Likes and 595 million Twitter followers. Last week, those brands added 18.5 million new Likes and 4.5 million new followers — so on a percentage basis, their following grew 55 percent more quickly on Twitter than it did on Facebook. Now, you might quibble about whether pitting Facebook Likes against Twitter followers is a bit of an apples-and-oranges comparison, but those are, ultimately, the main ways that businesses can count their following on each service. You could also point out the Twitter audience is still smaller than it is on Facebook — so even though Optimal said Twitter grew more quickly, the brands in question actually got more Facebook Likes than new Twitter followers. There are cases, however, where brands have a larger following on Twitter, full stop. Facebook-owned Instagram, for example, added the most Twitter followers of all the brands tracked — 279,500 new followers compared to 214,300 Likes. It has 21.3 million followers total and 4.6 million Facebook Likes. (Facebook itself came in at No. 3 on Twitter growth, adding 167,400 new followers and 217,200 Likes.) Not that Optimal CEO Rob Leathern is really trying to pitch this as Twitter overtaking Facebook. "I think it does show that Twitter is growing and becoming more relevant for brands, too - in a sense 'catching up', but also it is different as well," he told me via email. "A smaller but often more active audience." Optimal also broke down the data by industry. Department and general merchandise stores had the highest growth rate on Twitter (2.01 percent, versus 0.59 percent on Facebook), while books and magazines had the lowest (0.46 percent, compared to 0.61 percent on Facebook). Leathern said this isn't necessarily the first time Twitter has outpaced Facebook (in this very specific measurement) — it's just "the first time we are looking at the data this way."