The holidays could not be merrier at Facebook.
Not only does Instagram now have more users than Twitter, but the Palo Alto, California-based company's shares are up about 50 percent year to date. Now, for Facebook to carry its momentum into 2015 the key factors are mobile and advertising, Colin Sebastian, a senior research analyst at Robert W. Baird, a Milwaukee-based financial services firm, told CNBC on Tuesday.
"Facebook is really at the epicenter of a massive and secular shift to mobile," Sebastian said in an interview on "Squawk Alley." "That will certainly be the predominant force for growth in 2015."
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He added the company has already established itself as "the most legitimate mobile app form at a huge scale."
Advertising, the other major factor in continuing Facebook's momentum, could benefit the company as advertisers are shifting money from TV ads to online. "The slow-melting ice cube of TV ad spending is really starting to accelerate, and Facebook is in a very good position to capture that (money)," said Sebastian.
Facebook could also benefit by advertising outside of its own application, he added.
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There are some factors that could derail the company's momentum, though. One of these factors is increasing competition from other social networks, Sebastian said. "Facebook faces many regional social network competitors in various countries."
These competitors include Google+, South Korea-based Cyworld and Japan-based Mixi.
Advertising competition from Google could also hurt the company's momentum, as advertisers are not shifting money from Google to place it into Facebook, Sebastian said. "Competition is going to be for the incremental ad space online," he said. "In that case, Facebook will grow faster than Google, but both platforms are positioned to gain share."
—CNBC's Ken Underwood III contributed to this article.