If you are involved in training other colleagues on using social media, or arguing with your boss about the merits of investing your time (and money) in these tools, or maybe just if you are of a curious turn of mind, then it’s always a good idea to have the latest usage and demographic stats at your fingertips.

So here’s a round-up of the stats available today (3 August):

1. LinkedIn. DigitalBuzzBlog gives some 2011 figures for the network of professionals: 119m users, of which over 50% are male and 70% are aged between 25 and 54. Read the whole set of stats yourself: LinkedIn: Demographics + Statistics 2011.

2. Twitter. As of yesterday, Twitter was reporting that 200m tweets were being sent every day across its network (thanks to New Media Age for this info, and the accompanying titbit that Twitter has recently been valued at $8bn, heavens to Betsy).

3. Facebook. All Facebook reports that over 80% of (I think, US based) advertising agencies used Facebook as part of their ad campaigns in the second quarter of 2011. This trumped usage of Twitter (39%) and YouTube (36%). Read more: REPORT: 81% Of Social Advertisers Tapped Facebook.

Also on the subject of the mighty FB, Rachel Beer (Beautiful Things) gives us a link to this infographic which breaks down the time spent on Facebook by the ‘average user’ into different activities – with looking at the homepage or newsfeed just topping the tables in terms of time spent. See the infographic here: Share of Time Spent on Facebook.

4. Mobile usage. Take a look at this useful infographic, charting the rise and rise of the mobile internet. Some of the juicy stats from the graphic include: there are over 1bn smartphones in existence, and half of all local searches are done using a mobile device. More here on Digital Buzz: Infographic: Mobile Statistics, Stats & Facts 2011.

And, to round of this post, take a look at this excellent article by Jennifer Whitehead (The Wall Blog) which contains a link to a useful infographic about how much our supposedly free social media work costs in real terms… and gives some UK-based analysis of what this might mean. The real cost of social media.