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If they're not playing, they're probably not into the game. Although sometimes it's just a chain reaction that gets out of hand and it's easy to correct. But if it's clear they're not into it, I just ask them if they want to stop for the day and if they don't, then we all regain our focus. If it's recurring disinterest in the game, then there's not much hope for the campaign I suspect.

I use the "Call" Rule:

The Call Rule: This rule allows for jokes, movie quotes and questions on the odd minister's sexual preference. As soon as any player or the GM believes things are getting too far away from the playing of the game or that things are just taking too long, he simply says “Call.” The player who is talking at that time must roll 1d10. If the number is even: They are fine, nothing happens. If the number is Odd: Their character does or says what the player has just said.
Of course this rule has its limits, statements like, “pass the chips,” “where's the cola?” and “can you get your children out of the room?" should not reason to enact this rule, and any player doing so should be looked at very sternly.

They might not be into the game. Perhaps it needs to be adjusted to be interesting to everyone. I like the idea of the most distracted players being placed at the front of the story or at least one adventure, so they feel the connection.
Also, having a pizza party beforehand or promising one afterwards might be a good way to get all the socializing done outside of the game session.
OR, make a game session that is difficult to get to. The busy people who arent that interested will not make the effort to show up when they'd rather be working out or shopping or whatever. Let them show up to the occasional session when they really feel the urge to participate.

The call rule is a favorite of mine, though I never used dice, hmmmm....
I've looked back on a few sessions as of late and realized we spent 4-5 hours just laughing and somehow we got through the scenarios and was able to get some game time in. Its a little disruptive and quite aggravating for me as a GM at times but with all the stress of real life, its been a great stress reliever. When it does get way out of hand I have a reliable player that I just sit and stare at blankly and he has an amazing ability to get everybody focused. Don't know what it is but all of the sudden the air and mood shifts and most of everybody starts getting back on track. And if that doesn't work and the Call doesn't work...i break out the potato cannon and load it with combat dice (the ones that have the sharp points, the D4 is effective with or without the cannon, and doubles as caltrops when pulling a player away for some one on one action and you want to keep eavesdroppers at bay or disabled).
And then there's the GM that pulls everybody back in with some ludicrous thing that has just happened in the game: A giant space cow plummets to earth and hits a player, killing him instantly and damaging other characters within the blast radius of the meat bomb. Of course this doesn't really happen but its effective a few times if used sparingly to bring everybody back.