Me and my boyfriend have been dating for about a month although we've known each other for almost 4 months?

[Sonu]

The thing is he's kinda socially active while I only talk a lot to few close people. He does have friends that are girls and I'm totally cool with it since I do have guy friends too! But there's this one girl I kinda dislike although he claims she's like a sister to him. I did mention it once to him that I wasn't comfortable with it and he reassured me I'm the only girl he wants. He did tell me he messages her almost everyday on facebook. But I notice that he's constantly liking her profile pictures whenever she changes it and he likes each and every status of hers, not to mention he likes her Instagram pics too. And although I did mention once of how he likes every picture of hers but sometimes even fails to notice I upload mine, he again reassured me there is nothing more than 'friendship' between the. I don;t like where this is going. I don't like the fact that although he kinda knows I don't like her he does stuff like 'liking her pics on facebook'. What should I do? Talk to him about it? I just can't ignore it! Please help me