SEGA released a special collaboration LINE messenger sticker set for FREE the Love Live! x SEGA Hanamaru & SONIC one. You can get it until October 2, 2017 after that the set going to disappear.
If you want it, visit this link (https://line.me/R/ti/p/-SPYS7OuHB) and add the SEGA to your friend list using the QR code. Than in the auto message you will find the link for the sticker set. Sadly the sticker set is not available in all regions. In the gallery you can find the sample picture of the set.