Despite being one of the prominent new features introduced in Pokémon X and Y, Mega Evolution was all but ignored in the series’ latest installments, Pokémon Sun and Moon. Players could still Mega Evolve their Pokémon in the new titles, but only after acquiring the corresponding Mega Stones from the Battle Tree, which could only be accessed after clearing the main quest — and even then, not every Mega Stone was available for purchase. Fortunately, The Pokémon Company has confirmed that the missing Mega Stones will be distributed to players as gifts for participating in events.
The first Mega Stones to be distributed will be Mawilite and Beedrillite, which will be given to Sun and Moon players who register for an upcoming online competition in February as an entry gift. According to The Pokémon Company, other Mega Stones will be distributed in a similar fashion, so it looks like players will need to be active on the Global Link if they hope to complete their Mega Stone collection.
Source: Serebii.net