FORTUNE — Microsoft is snatching up business social networking tool Yammer – a.k.a Facebook  for the workplace – for $1.2 billion.  This acquisition hints at a major shift in the way the planet’s largest companies use and choose software.

The Wall Street Journal notes that the Yammer purchase shows “Microsoft may be trying to plug holes in its ubiquitous Office software.”  It goes much farther than that.  Enterprise software – the tools used by companies to do everything from process payroll to monitor customer leads and make spreadsheets – is right now in the midst of an unprecedented renaissance and “full-blown reawakening,” according to no less an authority than Peter Levine, general partner at Andreessen Horowitz, the venture capital gurus that have invested in Facebook (FB), Twitter and Zynga (ZNGA).

The proof?  In just the last month, enterprise giants including Oracle (ORCL), Salesforce (CRM) and now Microsoft (MSFT) have shelled out $2.25 billion for social business upstarts like Vitrue, Buddy Media and Yammer.  The spending spree reflects the fact that enterprise software is swiftly evolving and tech’s titans are fighting to keep up.