Rumors are that Google made an attempt, Facebook admits a recent attempt, and Amazon founder Jeff Bezos already invested last June (2008).  The most recent investment is reportedly

TechCrunch reported on 1/24/09 that Twitter turned down a $500M purchase offer. 

In mid-February a $35 million investment in Twitter at a $250M valuation made headlines.

On 3/4/09 Business Week did a feature article on the red hot 3-year-old, and that same day Motley Fool speculated that Eric Schmidt’s recent comments could mean Google will acquire Twitter soon.

The latest investment supposedly was too attractive to resist.

Twitter reportedly took in about $15 million back in June, 2008.  With the recent $35M and prior investments that comes to about $55 million invested so far.  Stay tuned to see if Motley Fool’s prediction is true.  Personally, I think they will be acquired by Google, but not so quickly.  Co-founder Evan Williams already sold his last business, the famous Blogger, to Google so he’s been there and done that.  He doesn’t likely need the money, but they know each other well.  I predict Twitter will hold out, grow as fast as possible and will probably take something in the range of a $1 to $2 billion offer within 18 months.

Twitter Has Potential Buyers Atwitter - Business Week - 3/4/09
As the microblogging service explodes in popularity, the suitors are lining up. Facebook has already made a play—others will likely follow.

Twitter Gets Help from Bezos - Business Week - 6/26/08
Funds from the Amazon founder and Spark Capital will help the “stressing” microblogging outfit rebuild its technology

An index of Business Week articles about Twitter can be found here.