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Early adopters may love to check in at nightlife hotspots on Foursquare and Facebook Places, but what might location technology look like once it moves into the mainstream? Fast-growing startup Life360 is aiming for that mainstream market and has interesting plans. The company offers a free app for family members to track each other's locations.

The in-part Google-financed startup announced a big jump in growth this Spring when a wave of media coverage lead it from 1 million users to 2 million in just 10 weeks. Now the company tells us it's hit 3 million in 9 more weeks and this week it announced a big new feature: carrier-level location tracking of any phones, including feature phones, for free. Life360's plan is to undercut the market on location data, scale up fast with a fremium model and then build its business with value-added services that incumbents in industries like home security and insurance are offering today with super-high margins.

Location-based "check-ins" - are they hot - or not? Two days ago we ran a guest post by Mark Watkins, CEO of mobile startup Goby, who argued that check-in services were unlikely to thrive and that startups had to do something more.

Today, though, family app Life360 announced it just hit 2 million families using its service - 1 million of whom started using the service in the last 10 weeks. How did they do it? They re-imagined the check-in. While other services let users declare what venue they are at but leaving most people wondering why they would want to do so - Life360 offers a clear and simple value proposition. Check-ins on that service send simple GPS co-ordinates from a child to their parents. "I want you to check-in on that app throughout the night so I know where you are and that you're ok!"