Microsoft Corp.’s $26.2 billion acquisition of LinkedIn Corp. gives new life to a social-networking pioneer that has built a huge user base of mostly white-collar professionals but has struggled to maintain its prominence in a competitive and fast-changing sector.

Since co-founder and Executive Chairman Reid Hoffman initiated LinkedIn 13 years ago by inviting 350 of his contacts to join, the company has grown to claim 433 million members, including 105 million active monthly users.

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