It seems to me that there is a small but steady decrease in number of games played. For example, 48 million games were played in August 2014. Comparably, there were only 32 million games played in August of 2015. That being said, keep in mind that the absolute number of games played may not be the best indicator of the game's health or popularity because:
1. It is possible that games in Season 5 are longer, so fewer games are played for comparable hours of entertainment.
2. It is possible that a lot of the "extra games" in Season 4 were due to Dominion bots.
3. Riot has increased monetization of their game with ward skins, icons, chromas, and various skins at higher price tag (1350RP+), so even with fewer players and less play time their revenue may actually be growing.
In case you cannot view the interactive chart below, try the imgur image (non-interactive).