Billabong  Suggested by simrun_lata 

It's custom. http://www.underconsideration.com/brandnew/archives/hand_beats_filter_in_new_instagram_wordmark.php#.UwGGMkJdVFk
However, it might be based on the old font that they used: Billabong.

The closest one is the one below but if you want to download it free, then search: HoneyScript on Dafont.