WordPress twenty fifteen theme : Add Social Media Icons/Buttons :
Twenty Fifteen allows you display links to your social media (icons) profiles, like Twitter, Facebook and Google plus.With twenty fifteen WordPress theme, social media sites will automatically display its icon in your social menu.You need not to use any social media WordPress plugin to display your social media profiles, buttons/icons.
1. Create a new Custom Menu, and assign it to the Social Links Menu location.
2. Add links to each of your social services using the Links panel.
3. Icons for your social links will automatically appear for the following social media sites.
Available Social Media Icons in twenty fifteen theme:
1.Twitter
2.Facebook
3.Google+
4.RSS Feed
5.Github
6.YouTube
7.WordPress
8.Codepen
9.Digg
10.Dribbble
11.Dropbox
12.Flickr
13.Foursquare
14.Instagram
15.LinkedIn
16.Email (mailto: links)
17.Pinterest
18.Pocket
19.PollDaddy
20.Reddit
21.Spotify
22.StumbleUpon
23.Tumblr
24.Twitch
25.Vimeo
Here's a detailed YouTube video to add social media buttons/icons to twenty fifteen theme.