By contagious I mean making a person feel happier and more likely to smile themselves.
Yes. Emoji smiles are contagious just like real human smiles.
As this study suggest the brain activity observed when people look at emojis is the same when we look at real faces, and subsequently mimicry and emotion contagion follows.
We presented 20 participants with images of upright and inverted faces, emoticons and meaningless strings of characters. Emoticons showed a large amplitude N170 when upright and a decrease in amplitude when inverted, the opposite pattern to that shown by faces. This indicates that when upright, emoticons are processed in occipitotemporal sites similarly to faces due to their familiar configuration.
Also, a more general study for on-line expressed emotions demostrated emotion contagion similar to face to face comunication.
We show, via a massive (N = 689,003) experiment on Facebook, that emotional states can be transferred to others via emotional contagion, leading people to experience the same emotions without their awareness. We provide experimental evidence that emotional contagion occurs without direct interaction between people (exposure to a friend expressing an emotion is sufficient), and in the complete absence of nonverbal cues.