crv-logo.pngMark Jacobstein, a serial entrepreneur, is leaving a top position at Loopt, a mobile social mapping company, to become an entrepreneur-in-residence at Charles River Ventures.

The move is surprising because Silicon Valley’s Loopt is widely seen as an exciting young startup, having announced a major distribution deal with Sprint in July — the first ever mobile social site to land a deal with a national carrier.

Loopt lets friends share their location and status with each other, send messages based on proximity to each other, and view photos and other information tagged according to geographic location. It uses the GPS capability of some Sprint phones to automatically updates a user’s location every 15 minutes (our previous coverage).

When we visited Loopt last year, Jacobstein was actively representing the company, along with co-founder Sam Altman, a 22-year-old. Formally, Jacobstein was executive vice president of corporate development and marketing, and he joined the company after it raised a $5 million round from Sequoia Capital and New Enterprise Associates. He was considered “adult-supervision” for the company, brought in by the VCs. Sequoia, in particular, is known for backing young founders, and then pressing to bring in adult supervision when it fears younger founders can’t take a company to the next level (partner Michael Moritz once threatened to pull Sequoia’s money from Google when the young co-founders dallied on finding a new CEO).

However, Altman appears to have proven he is up to the task of running the company. Jacobstein says Loopt “is a potential rocket ship and one of the more interesting companies in the Valley right now.”

Jacobstein has his own successes under his belt, and he tells us that he really wants start and run a company himself.

He previously co-founded mobile game publisher Digital Chocolate. Before that, among other things, he founded, ran and finally sold online fantasy sports company Small World Sports to Paul Allen’s The Sporting News.