co-founder and chief executive

When Dr. Kelvin Leshabari was studying in 2008 for his medical degree in Dar Es Salaam, Tanzania, he felt isolated from medical researchers in the rest of the world. But then he stumbled upon a U.S.-based Web site, ResearchGATE, that takes the social networking concepts underlying popular services such as Facebook and LinkedIn and applies them to the research community. Leshabari, now 26, was able to connect with scientists in Germany, Israel, Canada, and the U.S.

Movieclips.com, an online movie clip-sharing service, launched in beta test mode on Wednesday touting the cooperation of six major Hollywood studios amid an industrywide slowdown in DVD sales.

The founders and private backers aim to make movie clips searchable and sharable and earn referral fees for viewers who go on to buy or rent movies via links that lead to Amazon.com or Apple Inc.'s iTunes.

After exiting the beta period, the company also hopes to put advertisements over the bottom third of the clips and share ad revenue with the studios.

The review Web site Yelp, which has garnered some criticism from the businesses put under its microscope, will soon let those businesses and others respond publicly to customers' critiques.

In an e-mail sent to some of Yelp's most active users Thursday, the San Francisco-based company solicited feedback about an upcoming feature that will let businesses post replies to user reviews. Businesses must first register for a free business owner's account. After a business posts a reply on the site, Yelp will alert the reviewer by e-mail.

When Facebook signed up its 100 millionth member last August, its employees spread out in two parks in Palo Alto, California, for a huge barbecue. Sometime this week, this five-year-old start-up, born in a dormitory room at Harvard, expects to register its 200 millionth user.

That staggering growth rate -- doubling in size in just eight months -- suggests that Facebook is rapidly becoming the Web's dominant social ecosystem and an essential personal and business networking tool in much of the wired world.