Facebook is a behemoth. But because it’s already so huge, its growth is slowing and is going to get even slower of the next couple of years. Twitter’s growth will also slow, but will remain much higher than Facebook’s, according to a report from eMarketer.

Over the last year, Twitter grew its user base 31.9%. Facebook only grew their 13.4%. This is a flip-flop from 2010, which saw Facebook growing at a staggering 38.6% and Twitter at 23.5%. So, according to eMarketer, 2011 marked the first time that Twitter overtook Facebook is user growth.

Although both growth rates will fall over the next couple of years, it appears that Twitter will continue to grow 3-4 times faster than Facebook. Check out the growth graph below:

According to eMarketer, Facebook just doesn’t have anywhere to go, while Twitter could increase it’s user base to 37.6 million (monthly users) by the end of 2014:

Twitter’s size, which is fairly small, is one factor that makes such growth rates possible. Facebook already reached an enormous audience of nearly 133 million US internet users at the end of 2011, a figure that will surpass 150 million by 2014. Twitter, in comparison, had a US user base of less than 24 million at the end of last year. Still, between 2010 and 2014, eMarketer predicts, Twitter will about double its US user base, reaching 37.6 million microbloggers by the period’s end.

Of course, the forecast is only talking about Twitter’s growth rate exceeding that of Facebook’s. When it comes to active monthly users, Facebook is still the king – by a longshot.