Is Facebook speeding up while Twitter slows?

On the heels of Twitter’s latest fundraising round valuing it at a reported $1 billion, data suggests the site’s growth may be slowing. Twitter.com’s marketshare of visits peaked in June then declined into this month, according to analytics service HitWise.

The deceleration would also come as Facebook’s user growth speeds up — the rival saw 300 million users this month, accumulating 50 million users in two months’ time. After copying Twitter features like the site’s stream of updates and user mentions using the ‘@’ symbol, Facebook’s management said that its competitor was in the “rear-view mirror” last week.

“To focus on a company with 40 million users that is not growing is not a good idea,” said Facebook’s vice president of growth, mobile and international expansion Chamath Palihapitiya.

Ouch.

One caveat: Hitwise’s data only looks at the site Twitter.com, and doesn’t count mobile or application traffic. Since about 80 percent of users do not visit Twitter.com to use the service and either use their phones or other clients, using the Web site URL to gauge the service’s growth is far from perfect.

So Hitwise pointed to search volumes for Twitter as further evidence (below the first chart).