Twitter had 'around 300' advertisers in the UK in 2012

"Twitter has compiled a top-10 list of the best promoted product campaigns by brands on the platform this year, based on a combination of data including total engagements, reach and impressions.
[...]
Twitter started serving ads in the UK in September last year. This year it ran around 300 campaigns in the region.
Top 10 UK Twitter Campaigns
@EE, brand launch campaign, by MEC (October)
@StarbucksUK, #FreeStarbucks campaign, by Mannigottlieb OMD (March)
@Puma, Usain Bolt's 100 metre final campaign, by Zenith Optimedia (August)
@DairyMilk, sample giveaway for puzzle solvers, PHD (September)
@EASportsFifa, to promote the launch of Fifa13, MediaCom (September)
@EnterTheCabinUK, Lionsgate promotion for launch of the film 'Cabin in the Woods' by MEC (April)
@CokeZone, #freecocacola trend, by Carat (September)
@AdidasUK, Olympics closing ceremony campaign, by Carat (August)
@NowTV, Sky’s launch campaign, by MediaCom (August)
@AssembleLive, Disney Pictures promotion for launch of The Avengers movie from Disney Pictures, by Carat (April)"

Source:  Marketing Magazine, 13th December 2012