Use this expression to talk about someone who "puts on an act". In other words, someone who pretends to be something that they're not. For example:

He likes to act like he knows everything.

She likes to act like she's so innocent, but you and I both know that's not the case.

You like to act all big and tough. Please! You're a total softie.

Talking about people this way sounds negative. In the example above, the wife might be annoyed that her husband pretends not to know about the social network.