This article provides an introduction to the use of altmetrics as a tool to assess research impact. In particular, it looks at the evidence behind claims that altmetrics allow the impact of research to be measured in days rather than years. Low correlations between altmetrics and article citations make it doubtful that altmetrics can reliably predict future citations. In addition, there are good reasons to qualify statements that altmetrics measure the wider impact of research on society. Librarians should be careful not to overstate the value of altmetrics when recommending their use as a complement to more traditional measures of research quality.
Altmetric measurements derived from the social web are increasingly advocated and used as early indicators of article impact and usefulness. Nevertheless, there is a lack of systematic scientific evidence that altmetrics are valid proxies of either impact or utility although a few case studies have reported medium correlations between specific altmetrics and citation rates for individual journals or fields. Finally, the coverage of all the altmetrics except for Twitter seems to be low and so it is not clear if they are prevalent enough to be useful in practice.