Market research firm eMarketer on Monday reduced its usage estimates for Facebook users under the age of 25 - an issue that some competing social media platforms aren't expected to face.

Facebook, the world's largest social network with more than two billion monthly users, is expected to see usage among teens and young adults slow. In the coveted 12 to 17 age group, the firm anticipates Facebook's monthly user base will fall 3.4 percent down to 14.5 million people in the US.

If the estimate holds, it'd represent the second consecutive year of expected usage declines by this age group. Last year, Facebook usage by this demographic dropped 1.2 percent.

Declines are also expected in the under 12 and 18 to 24 age groups.

Teens and young adults aren't abandoning social media but rather, seemingly spending their time on other platforms.

eMarketer has increased its projections for Snapchat usage which is now expected to grow 25.8 percent to 79.2 million monthly users. Likewise, Instagram (which Facebook owns) usage is forecasted to grow 23.8 percent this year to 85.5 million users.

Oscar Orozco, a senior forecasting analyst with eMarketer, said that outside of those who have already left, teens and tweens remaining on Facebook seem to be logging in less frequently and spending less time on the platform. At the same time, there are now "Facebook-nevers" - those that are overlooking Facebook altogether.

Snapchat and Instagram, Orozco added, have found success with younger demographics partially because they are more aligned with how they communicate---that is, using visual content.