New data from eMarketer dives into mobile usage of Twitter and Facebook, demonstrating the growing importance of mobile for social networks.

According to the eMarketer research, its first-ever stab at estimating Twitter usage on mobile phones, 28 million people will use Twitter on their mobile phones at least monthly this year, up 22% over 2012 figures.

Despite this growth, penetration of the overall population is low: Twitter mobile users will account for just 11.4% of mobile users and 8.8% of the overall population this year.

What’s more, Twitter’s mobile penetration rate is forecasted by eMarketer to actually fall in coming years, due to the expected overall growth of mobile phone social networking, especially on Facebook. Look at the difference:

eMarketer estimates that 67.6% of Facebook’s US audience will access the network on their mobile phones this year, rising to 95.5% by 2017!

The large gap in those figures makes more sense in light of the parallel gap in audience size between Twitter and Facebook, the latter of which is still a behemoth in comparison.

Since the overall Twitter population in the U.S. is estimated to reach 36.8 million users this year, that means 76% of all U.S. Twitter users will access the service via mobile phone, rising to 87% by 2017.

Do you use Twitter on your smartphone?

(Source: eMarketerMobile image via Shutterstock.)