although the overall users bases are totally different, the rest of the numbers concerning the two services are strikingly similar. it is interesting to note that many more people tend to log in to their facebook account regularly.
also, it’s of great interest to note that if you are considering advertising via either of the services — though fewer people follow brands on twitter than on facebook — those who follow on twitter are nearly 18 percent more likely to purchase something from that brand than those who follow a brand on facebook.